What are his communication skills like? My son, now 20 went through a horrible period of biting when he was a little older than your son. At a check up I mentioned it to his doctor who referred us to a speech pathologist. Within a few weeks of speech therapy the biting stopped. He had a really large vocabulary but was omitting random sounds in words. Because he was so young I didn't recognize that he was having a speech issue, I just thought that's how babies sound & as his mom I knew what he was trying to say. The biting would happen when he tried to interact with other kids & when he wasn't understood he would bite out of frustration.
Best of luck