I wish I had some magic way to tell you how to put a stop to this, but I don't.
My son is now 29, but when he was around your son's age, he too was a biter. I tried everything I knew to get him to stop....explaining how much it hurts, time outs, and even a few smacks on his butt, but nothing seemed to get the point across.
One day we were in the supermarket, and I had him in the seat in the cart, and he kept taking things off the shelves when I told him "no". He got angry, leaned over, and sank his teeth into the fold of skin between my thumb and index finger, and bit until I bled. That was it......I'd had enough. I picked up his little arm, and bit him back. Not hard enough to break the skin,or leave a bruise, of course, but hard enough to make him cry. Then, when he calmed down, I told him "that's how it feels....don't ever do it again", and from that point on, he never did.
I know that seems a bit drastic, but sometimes that's the only thing that they understand at that age. I felt very guilty about doing it, but when I discussed it with his pediatrician, the answer that I got from him was...."well, whatever works", so I guess I wasn't too far off base. If nothing else works, give that try!