How do we stop him?

Lots of people will have lots of good advice, but I applaud you in starting to socialize your dog - I think it is SO important. That said, have you tried a dog day care? There are a lot of really great ones out there that will watch your dog (and the other dogs) and let him socialize in a safe environment.

Some I'd recommend are Puppy Playground on 65th street b/c Allisonville and Binford, TL Pets Doggy Daycare at St. Clair and Capitol and there is a new one in Irvington (animus.us i think) and the owners are animal behaviourists as well.

Good luck!

I hope you get a lot of advice on this. Outside of a doggie daycare (where they keep an eye on them all the time), you do have some options. I would praise him in a loud, happy voice everytime he plays correctly, and stop the play when he's playing in a manner you don't like. I have to say, though, that lots of dogs play loudly with growls and even mouthing. As long as all of the dogs are having a good time, I really wouldn't worry about it.

Agreed. Nigel plays rough - wrestles and mouths. That's just how he plays. He usually only plays like that if the other dog is a wrestler/biter too - but it DOES look like they're going at it!

It looks like Nigel and Mathis would play well together :)

Thanks for your responses! It's not the growling and barking that really bothers me, it's the baring of his teeth. That's normally in most dogs a sign of aggression and although he doesn't mean it like that, I think it scares other people because he's so big. He always reads the other dogs' reaction and he always plays nice (no biting or clawing), so I know he normally is ok.

You were very responsible to take your dog out of the dog park when you knew he was acting up. Some dog owners just don't care.

Has Mathis been in any formal obedience training, like in a classroom setting with other dogs? That would be a controlled, safe environment for him to learn right from wrong.

Good luck and keep working wih him, it will pay off.