So he's doing the end-of-feed nip?
Can you see it coming at all? There's usually a 'tell'. If he's well attached with the nipple back in his mouth, he'll have to pull back a bit before biting and this is when you can quickly pop him off before he does it.
If you can't tell, it may be helpful to go back to basics with positioning and attachment and make sure he's in really close, big wide mouth, etc, etc, so that he'll have to pull back a bit to bite. Once babies get bigger they can often feed quite effectively without drawing the nipple in to the back of their mouth and so they can get these sneaky bites in. Paying close attention to P&A can help a lot.
Hope you can get him to stop soon!