It is typical boy's game, but...you're not another colt, you're the lead mare. Act like it! First, NO handfeeding or playing with his mouth. If he just as much as THINKS about nipping, go after him, chase him away, make yourself tall, growl, do whatever it takes to send him in retreat. Be consistent. He will understand quite fast if you mean business.
I never had to do that to 3 year olds. My colts, within their first week, would get slapped on the side of the neck for it and none of them has ever thought about trying ever.