I recommend getting one of those large collar things, the ones that go around his head.Leave it on for a while and when he sees he can no longer bite his tail he will stop trying.Or, you could use a muzzle.
How Do You Stop A Dog From Pulling His Hair Out Of His Tail?
Some dogs bite or chew at their tails when they get bored, it's also worth checking his anal glands because that can cause irritation if they're full.