Cozie, I recall you saying that Senior Boy was a bit bitey when he first came to live with you. If Little M's naughty, and I hold my hand up (like STOP), he just launches himself at the hand, or slaps me. Same as if I point a finger at him if he's being an arse, he takes it as an invitation to bite my finger. Just now, I was tapping on the keyboard, and he walked towards me, jaws open like a crocodile.
He never breaks skin though.
How did you get Senior to stop nipping? He gets playtime and has lots of toys.