Rather than "biting him back" (which may work; or it just may lead to aggression, which a new study on the "Cesar" type techniques suggests), follow this link The Bite Stops Here

It isn't a quick fix but it will work and is less likely to cause problems.

To stop pup biting his lead in a non-confrontational way without jerking and possibly hurting him, either get a chain lead (dogs tend not to like metal things in their mouths) or invest in the cheapest lead you can get hold of, and at the point where he gets hold of it, coat it in Vick Vaporub.