Is he gelded? Stud colts are often very mouthy and 'selfish', so to speak.
IF HE BITES: PROMPTLY. The SECOND he does it, SMACK HIM. Hard. On the nose. Don't just flick him and go "Oh stop!" give him a good whack and go "NO."
He will be very surprised and offended at this. I punish for the behavior, let it sink in a second, then rub them on the forehead or between the ears. Act as if nothing ever happened.
Your not going to injure your colt doing this :)
Consider it this way, if he where to bite him mother, or even an older gelding, they would likely turn around and chomp right back at him, or even send a hoof in his direction. One liable to connect.
I had the same problem with a stud colt, it got better after he was gelded. It took maybe, two times? For him to understand that no, I am not edible, and yes, I am bigger and stronger then you.
Even after gelding he attempted a little nibble, but a loud "NO" stopped him right in his tracks without me so much as touching him.
Youngsters are quite difficult to deal with, they have attention spans the size of a flea :roll:
Wait! I'll fix it....