You might try finding some books for kids his age that will help him. Did he do anything to startle the dog or did it just bite for no reason? Kids can startle without meaning too, I am putting NO blame on your son. Have you taught him to stand still and hold his hand out to a dog he doesn't know ( I don't mean walking up to one on the street but if you go to someone's house) That way the dog gets a sense of smell of him. And you might talk to him about that dogs bite for many reasons but that doesn't always mean they are awful dogs. Maybe restart him with getting aquainted with smaller dogs, ones you are sure won't bite and stay by his side the whole time while he interacts with it. It will take him a while to get over his fears. I'm really sorry that happened to him.
at 3:03 AM on Sep. 1, 2010