[ladyjane]

Agree with the last two posters. He needs a leash at all times so that you can pull him back to you if there is an issue. I would for now not take him where there are larger dogs. As for dog trainer suggestions by some, I much prefer behaviorists. You need to figure out what is going on with him and work with him to correct it. FIRST thing though is to protect him...while he thinks he is rough and tough, he is not and he could be seriously injured or killed with one snap from a larger dog.
Another thought....any time you see him start up, pick him up immediately and put him somewhere by himself. Let him know that his behavior is not acceptable. When he is then back around other dogs and behaving, praise him. Hopefully he will get the message.