Mouthing Problem continuing....

The "alpha roll" is a complete and utter myth. The only reason a dog would ever forcibly "roll" another dog like that is if the dog intended to KILL the dog being rolled. People who "roll" dogs are in grave danger of being badly injured-- because naturally the dog, thinking he's about to be killed, defends himself. So don't do it.
 
The submissive "roll" is something that is OFFERED by a submissive dog to a dominant dog. It is usually only offered in response to a very angry dominant dog, not something offered on a daily basis. If your pet dog is offering submissive rolls to you on a regular basis that means he is terrified of you.  Dogs exposing a belly to get a belly rub is something entirely different.
 
"mouthing" has nothing to do with dominance-- dogs play with each other by mouthing each other. If your dog is mouthing you, it means he's trying to play. Dogs have to be taught that mouthing humans in play is not ok.  And dogs are often noisy when they play. The siberians I see at the dog park make all sorts of weird growly noises-- it means nothing except they are having fun.

Oh goodness you don't have to see a behaviorist. Everything mudpuppy said is exactly right. Not everything your dog does that is naughty is about dominance. Chances are none of it is. Don't alpha-roll your dog. He's not biting now but if you keep that up, he might eventually.

Dogs do not have hands. When they want or need to handle something, they do it with their mouths. When they play, it is all about thier mouths. When they want to explore something, they do it with their mouth. Being mouthy is the default state of all puppies. When your dog puts his mouth on you in a way you don't like, immediately remove all attention to him. I've found that a really exagerated gesture like crossing my arms on my chest like a mummy and facing the wall (the dog doesn't know that that's boring for you) definately gets the point across that if you put your mouth on me, I ignore you. One of my dogs is a little mouthy and we were complete idiots and encouraged it when he was younger (playing rough) and in the past couple years I've worked on improving the situation. With the method of simply crossing my arms (he likes to nip at sleeves) and turning physically away from him, I've had great success.

Agreeing with mudpuppy and houndlove.  Have you tried saying "eh eh, no bite" in a calm, but firm tone and then putting a toy in his mouth and praising/playing?  My two used to get a bit mouthy when playing, and still do when rowdy guys come over and try to play rough with them. [:@]  By showing them what is ok to mouth on while playing, the behavior changed.  Now, when they really get excited, their first reaction is to get a toy, since they can't actually touch me with their mouths.  I looked after a Sibe briefly and she loved stuffy toys.  Does your boy like them?  If so I'd give that a try.

Also try freezing some of his chews and toys.  He's probably not out of the teething stage yet and offering something that soothes his gums will help to rediret the behaviour because it will be preferable to your hands and clothes.  Soaked and frozen rope toys are a good teething toy.  I offer mine the occasional frozen pigs ear or bone too.  Frozen Kongs are a great way to let him exercise his teeth and jaws and will be mentally stimulating too.  

Everybody's given some great advice on this thread.  I hope the OP follows the advice.

Allright, no rolling the dog over. The problem with the crossing arms thing is that he'll begin to bite the meat of my legs, or my butt, which hurt way more than the hand, and this guy's jaws are moderately strong. Oh yeah, also, I tried closing his mouth, but he'd just bite right after that, so... is it normal for them to still be mouthing at this age? (approx 14 months). And Chuffy, I seriously doubt he's still teething, from what I read they stop teething at 10 months, and he's way past that.

ORIGINAL: mudpuppy
Dogs exposing a belly to get a belly rub is something entirely different.

 
 
Yeah, he loves belly and chest rubs, so most the time, he rolls onto his belly, halfway for a rub.

Did you consider my toy suggestion?  I was sort of interested to see if you had already tried that method.
 

Oh yeah, also, I tried closing his mouth, but he'd just bite right after that, so... is it normal for them to still be mouthing at this age? (approx 14 months).

 
Closing his mouth by force is only going to get him more riled up, and more bitey.  Not the way to go.  Is it "normal" for untrained dogs to still mouth at this age?  Yes.  Age has nothing to do with it; these behaviors don't just magically stop on their own.  We had a 2-year old 170 lb St Bernard at our shelter who would take your entire forearm into his mouth.  He wasn't being aggressive, and was actually very friendly, he just hadn't been taught how to properly play with humans. 
 
A word to the wise: a young Sibe isn't going to be easy to work with when he is full of energy.  These are seriously high energy dogs, and at 14 months he is physically mature, but still has puppy enegry.  If he is driving you nuts trying to instigate play with his mouth/teeth, he likely needs more exercise.  Don't give up!  Sibes are hard work, but its worth it. [] 

when he bites you, let out a loud yelp noise before you do the Withdrawl of attention. Dogs tell each other that they bit too hard by yelping, so that should help. Also, it is very helpful to be pro-active-- initiate a fun game with the dog (with a toy for him to bite on) before he tries to initiate a game with you by biting. Watch for him doing something you like (sitting quietly? not chewing on the couch? ) and begin your game.
also siberians require LOTS of exercise. They were bred to run all day every day, so you have to really work at it to get them tired out. A tired dog is a good dog.

I tried Bitter Apple on my dog to stop mouthing, and that didn't do much either. He just learned to run away as soon as I'd reach over to grab a bottle. That's not good. And, when the taste wore off, he was back being a 'lil monster. So, forget about Bitter Apple-you don't want to rely on a foreign object to keep him from mouthing you.

The little growls that he makes while running are probably playful growls. Does he get down in a play bow with his front paws bended lower than the rest of his body? Sounds like he is inviting you to play with him, the doggy way, which is too rough for us. You dog has to learn that it's not appropriate to bare teeth into humans. Suggestions above sound good. Redirect him to what he is allowed to play with (chew and shake). Don't do alpha rolls, only a hand full of professionals can successfully do it at a right time with the right dog.

I found a good solution when you need to exercise your dog and it's too cold to take him out for a long walk - a laser pen. My dog loves to chase it. (You just need to be careful not to point the laser, or laser reflection on his face.) I use it when my dog is full of beans.

It's common for dogs to teethe up to 14 months, or even beyond.  In any case, you can't be sure that his gums are not paining him, so try freezing some chews for him and see if he likes them.  Freezing a Kong also makes it harder to empty which makes it last longer and makes it more stimulating for the dog.  Even if he is not teething, the same principle still applies - make his chews and toys more interesting and more rewarding to chew on to help you redirect this behaviour.

The problem with the crossing arms thing is that he'll begin to bite the meat of my legs, or my butt, which hurt way more than the hand, and this guy's jaws are moderately strong.

In that case I would simply walk into a different room and close the door. Stay there for a 30s to 1min "timeout". Let the dog know that by biting you , he is ending all play and causing you to leave.

All good suggestions, and I tried yelping and ignoring the dog. However I was sitting on the floor so he didn't really wanna bite my legs. I looked at the ceiling fan, and then he began barking, which is strange, because he almost never barks, not even when someone's at the door.

The problem with the crossing arms thing is that he'll begin to bite the meat of my legs, or my butt, which hurt way more than the hand, and this guy's jaws are moderately strong. Oh yeah, also, I tried closing his mouth, but he'd just bite right after that, so... is it normal for them to still be mouthing at this age? (approx 14 months). And Chuffy, I seriously doubt he's still teething, from what I read they stop teething at 10 months, and he's way past that.

 
I also adopted a 18 month old Siberian not long ago, and he's been moderately mouthy, though mostly in play.  I use a very firm "no bite" with him, and I've had success, though it's taken some time.  The good thing about Siberians is that they're extremely bright, and he's likely to catch on quick once he understands what you mean.  (Unfortunately, they're also quite stubborn, so you do have to balance the two.)
 
Siberians are very enthusiastic and affectionate, and usually they just want to play and have no idea that it hurts.  I tend to say "Ouch" very loudly when he bites now, and he immediately lets go.  It took awhile, though, so be patient--Huskies can be a real pain to train, but they're worth every moment.