Reactive in new situations

he lived in a foster home with 11 or so,and people I believe he hasn't been in a situation like this and I'm wondering how to do this correctly?I have another chihuahua/rat terrier mix who loves going to petco and that,enjoys meetting other dogs and
I'd love to give him the same

I don't understand the issue at hand.  Are you saying that he does not like meeting new people or is it a problem that he and your resident dog are having?[8|]
 
Can you describe his "reactive" behavior?

Oh sorry I posted it on another board with the problem in the post,wasn't getting any replies so I copied and pasted the content of the post here and for some reason changed the title.

No he (Jasper)and the resident dog are getting along well. It's in pet stores and the general public that we have a problem. Jasper lbarks/whines and lunges at the dog.People he's fine. So we took him to Petco today we decided to seperate them,it was the rescue Jasper's 2nd trip there,thiknking  they'd do better seperately thinking Jasper's guarding Tucker ,the resident dog,due to domince or otherwise and it wouldn't help his fear agression. We walked around the store,he was skittish but was doing fine otherwise,not a peep out of him other then the occasional whine. We run it to a dog and he's barking/whining bloddy murder and tries to lunge but I wasn't giving him much leash in the first place.We browsed around got what we needed while trying to avoid the other dogs at least with Jasper anyway. We'r about to go to the checkout with my mom carrying him which I told her wasn't the right thing to do because it would give him the idea that he wasn't secure on the ground ?when we came across the owner of a puppy in a cart with a employe petting him and evreything. My mom foolishly walked over talking to him soothingly with good intentions and brings him closer so he's touching noses with the pup. Jasper lunges,the employee smacks him in an attempt ot break him up,Jasper lunges at her and breaks the skin,. We leave horriefied and embarrased. We now realize it can't be handled on our own and are planning on calling a behavorist but is there anything else we can do?And my mom's talking about buying a halti ,I cringe at that ,correct me if I"m wrong but isn't it mainly for a dog that pulls and gives the owner more control on a walk?

A couple of things are jumping out at me.  First you haven't had Jasper very long so he is probably really nervous.  I would suggest a basic obedience class to help him gain confidence, build trust with his new family, and be around other dogs while on leash.  It is very different for dogs to approach while on leash vs. without.  I think a class with other dogs would be more helpful than a behaviorist at this point, but I wouldn't want you to hesitate to call one if it becomes needed.
 
I do think that giving each dog their own time is a good idea.
 
Next dogs don't meet each other nose to nose, so forcing them together like that is asking for problems.  Also, just like we don't like every person we meet a dog is not going to like every person they meet
 
 

And my mom's talking about buying a halti ,I cringe at that ,correct me if I"m wrong but isn't it mainly for a dog that pulls and gives the owner more control on a walk?

Yes, that is what a Halti is for.  I would suggest an Easy Walk Harness instead.  If a dog pulls with that harness on, he turns himself around toward the person holding the leash.
 
Easy Walk Harness:
>http://www.dog.com/itemdy00.asp?T1=100612+XS+BLK[/size] (our board sponsor)
>http://www.sitstay.com/store/equip/collars2.shtml[/size]    (half way down)
 
I echo Seashell's suggestion of a basic obedience class.  That will help with Jasper's confidence level.  You can also help his confidence by showing him that he has one or more strong pack leaders:
[linkhttp://forum.dog.com/asp/m.asp?m=298804]http://forum.dog.com/asp/m.asp?m=298804[/link]
 
[color=#000000][size=3]

[font=verdana][size=3]We run it to a dog and he's barking/whining bloddy murder and tries to lunge but I wasn't giving him much leash in the first place.  [/size][/font]

This is called "leash aggression".  Here are some resources:
 
[linkhttp://www.flyingdogpress.com/onldagg.html][color=#000000]http://www.flyingdogpress.com/onldagg.html[/link]   (scroll down)
 [color=#0000ff]
[size=3]McConnell, Patricia, Feisty Fido: [font="times new roman"][size=2][font=verdana][size=2]Help For the Leash Aggressive Dog[/size][/font][/font], 6/2003 (booklet)
[linkhttp://tinyurl.com/5vhqz]http://tinyurl.com/5vhqz[/link][/size][/color]
 
Brown, Ali[font=verdana][size=3], Scaredy Dog! Understanding and Rehabilitating Your Reactive Dog, 5/2004
[linkhttp://tinyurl.com/437q3]http://tinyurl.com/437q3[/link]
 
Aside:  [size=2]If you are concerned about the Easy Walk Harness coming off, use a coupler (on the end of the leash) attached to both the harness and collar. You can probably find a coupler at your local pet store. Pictures of couplers: [color=#0000ff]
[linkhttp://www.superleash.com/Super_Dog_Leash_Couplers.html][font="times new roman"][color=#000000]http://www.superleash.com/Super_Dog_Leash_Couplers.html[/link]
 [/color][/size][/color][/font][/size][/font][/size][/color][/size][/color]

Thanks for the advise but we've already called in a behaviorist..come to think of it we probably could have gone the route of obedience classes but I don't know if I'd be able to trust Jasper.Luckily I'm making a trip to the mall Monday so I'll look in to those books as well as the easy walk harness.

Thanks for the advise but we've already called in a behaviorist.

Let us know how it goes! It never hurts to do a little extra research beforehands anyways, so you can evaluate the behaviorist too and have a better background of information to understand what he/she is telling you. (Like going to the doctor knowing nothing about health, versus going to the doctor after going through med school and being trained in accupuncture. Quite different experiences!)

True. Actually after the behaviorist we're enrolling them both in basic obedience so yes I will keep y'all updated Actually I'm 16 and considering canine behavior as a minor at least so it'll also give me insight on that............