Dog Acting Different

I have a 6 month old Male Pit Bull pup. We have raised the dog since he was 3 weeks old. Everything was going great with his training until around 4 months. He nipped my 7 year old son when he tried petting the pup while the pup was playing with a toy. We worked on dominance training and everything seemed to be working well. The biting stopped and the pup was well behaved. About a week ago me and my wife had left for a couple days to have a baby. We hired a sitter to let him out and watch over him while we were gone. The sitter lost the dog and he was gone for about a week before a guy noticed the flyer we posted and brought him back. He is acting completely different. If we try to pet him he will growl and show his teeth. When I scold him for the behavior he walks off but tries to come back to me after a couple of minutes. When I try to pet him again he will do the same thing again. I've tried the advice about holding the dog still on his side until he submits but it hasn't corrected the problem. Any advice? Anyone know what may be going on with the pup?

First, has he been to the vet since he got back? I am wondering if he might have been injured while he was lost, and is in pain, which is why his behavior is different. Also, I'd not keep up with the dominance training, as you describe it, and go with a more positive approach. First get him a vet check, and be sure there's nothing physically wrong, and then start over with his training as if he were a new dog, redoing leash training, and basic obedience, you need to reestablish the bond of trust you used to have with him. Positive training, praise and toys for good behavior, no toys, no treats no attention for bad, but this behavior change is so sudden, it makes me think he might have been hurt.

Also, of course, you have had another big change, there's a new baby around, so everything's different in that regard, too. What's your pup's name? Just curious!

I agree w/ what Karen said.

Action plan:
1. Vet check
2. STOP "dominance" training - it's a very old model, outdated, has been proven NOT to work
3. Start positive reinforcement training. Take several classes w/ the dog.

Find a good place for classes, here:
http://www.apdt.com/petowners/ts/default.aspx

Do group classes, NOT one on one training.

Good luck!