My GSD's guarding obsession is Me!

It sounds like possessive aggression. I could be wrong be that is what it sounds like. I would seek help from an animal behaviorist or a dog trainer that has knowledge of possessive aggression in dogs.

 I'd like to put a stop to this behaviour asap, but am not sure the best way to go about it. Any suggestions? 

 
What are you doing when he behaves this way now?

Usually it's when I'm laying or sitting on the couch just talking with people, or watching TV. This morning I was in bed and my 13 yr old son came into the room, and he started barking at him. He didn't try to bite, or go to him, he just got out of his crate (I leave the door open) and started barking at him.

I have 2 other dogs, and he will not allow them to come near me as well. If I go to them and pet them it is ok, but once I'm on the couch, if they start walking towards us, he will stand up, and sure enough if they get within 2-3 feet, he will lunge at them as well. It doesn't seem like he is trying to hurt them, or anyone for that matter, he just nips at them to keep them away from me.

More info:

I've had the dog 2 months, I'm his 3rd (that I know of) owner. The guy I got him from said he barked too much.. That was the reason he was getting rid of him.

The first few days I had him, I took him to a pet store and he was aggressive towards all men, and other dogs. On our first trip to the Vet - the Vet would not go near him, and would not examine him even after I put a muzzle on him. I called a dog trainer. She got me started on clicker training him, and I have used only positive clicker training since. I had him neutered, and have worked with him on socialization skills. I take him to the dog park at least once a week now, he gets along with all dogs, and is no longer agressive to men. I take him on walks at least 2 - 3 times a week he is fine with other people and dogs coming up to us. He even allows others to pet him on our walks. He learned sit, come, heel, and stay in his first week of training, and we have now joined an advanced training class, and he will test for his CGC in a few weeks. He has the most *amazing* stay I've ever had in a dog. I train him off leash in a quiet cul-de-sac near our house, with some distractions, and we have had absolutely *no* problems. He really is a very good dog. He knows to wait at all doors and let people go thru them first, he will sit to have his leash put on or taken off etc etc.

It's just this one problem left - And I'm not quite sure about how fix it. He doesn't do it all the time, and rarely with family, but I do see him doing it more lately.

My suggestion is to get a copy of  Mine! A Guide To Resource Guarding In Dogs by Jean Donaldson.  I really like her books and intend to get a copy of this one in the future.  I enjoyed Culture Clash and The Other End of the Leash.
 
Sorry i am not more help! 

I think you're doing the  right things, at least it sounds like you're on the right track.  Since he's an obedient dog, perhaps when he begins barking at the "intruder" you should give him a command "Fido, Sit!" and get his attention back onto you.  Reward when he settles and follows the instruction. 
 
Also it sounds like he's got a good guarding instinct and you happen to be the lucky recipient of his guarding attention.  This is certainly behavior that you want to nip in the bud because it could escalate as he matures. 
 
I think that the biggest misconception among dog owners is that they don't correct their dogs in the appropriate manner because they think it will make their dog like them less.  As long as you are fair to your dog the dog will like you and stay bonded.  The most important thing is that your dog respects you as the alpha.  (this is a generalization not directed at you MC)   

Just wondering if your pup is fixed yet? He's hitting the adoescent stage right now doesn't sound like he's had that great of a start I would definately have this guy enrolled in an obedience class.