Change the words mouthy and nipper to “he bites” and you will change your attitude enough to change his behaviour. Sooner or later he will take it too far.
Pull the lead out of his mouth with a stern accompanying voice. Do the same for cross tie biting and all the rest.
Now, here’s where I get all controversial … the best success I had reforming a biter was to add in food treats to his training. Never let the horse ask or get pushy for the treats. And cut out tolerating any crappy behaviour when he is with a human. What he does in his paddock or stall is OK because it’s on his time. Give him stuff to play with. Let him carry stuff around on his time. On your time make it your time.
Good luck, it’s not easy, but you can change a biter into a reformed biter by being absolutely 100% consistent with your expectations.
