I don't know the right answer but I know the situation.
Our "Ducky" was born on 3/30 so about your colt's age and he is biting and like yours, it started when I'd pet / groom him and I think he's doing it to instigate grooming not to be mean but it's turning into a problem and he has teeth now.
What I started doing yesterday is to say "aaarrrggghhhh aaarrrggghhhh aaarrrggghhhh" (kind of like a machine gun sound a kid might make playing army) when he does it and reach and pinch him on the neck or shoulder like a bite from another horse. He seems to be getting the idea, but he also gets frustrated by not being able to bite-groom me. He reared up at me when I was sitting yesterday and I slapped his neck with my hand (not sure that was good but it was an automatic reaction).
Ducky's soooooo people friendly and I know how to deal with biting in an older horse, but he's "just a baby" and so adores people. This seems to be working, but like I said, not sure if it's the right answer. I've never really had this issue this bad before. My 2003 colt did some biting but not to this degree if I remember right. Because before yesterday, Ducky'd gotten to the point that his greeting to you is biting your legs and I figure this has got to stop before my parents and my sister and nephews come and visit!!! (Not horse people, and I think they'd think he's vicious!!!)