wanted to know why he started biting and how to get him to stop

Maybe something traumatic happened to him that frightened or hurt him, and he is still reacting to that. If it just started, give it a few days and see if he settles down. Try to keep him calm and see if he relaxes. If it has been going on for some time, check with your vet. His age might have something to do with it -maybe he is getting cranky.
It is hard to answer this question without more specifics.