R.I.
We went through the same thing with our son. We even tried lightly popping him on the mouth and he didn't care. We did a combination of using a preservative called Alum and we finally got so tired off it that we bit him back. I know it seems harsh but from my experience (I have 13 nieces and nephews and everyone has gone through it) this has been the only solution to it. Try the alum you can get it at the grocery store it tasty really nasty but is totally safe it is in the spice aisle.