This is a respect problem, not a training problem. My first suggestion
would be to geld him if he is still intact! Watch a mare and see how she
gains respect from her foal. The moment the foal disrespects mom she bites,
kicks, or slams it with her head. She is firm and not gentle. The secret
is in the timing. Your colt has kicked you. The quickest way to address
this problem is to carry a longe whip with you and the moment he starts
to get into position to kick, pop him, front or back, (the same area where
the show disrespect is coming from) at the same time yell NO! in an authoritative
voice. Do not hit him in or around the head as this can cause more problems
down the line.
My wife met with a similar problem one day in the open without a longe
whip, but fortunately she thought and acted quickly with what was at hand.
She and the colt had been side by side when the incident occurred.
Without missing a step when the colt initiated the kick with his back feet,
she grabbed a piece of old broken hose she spied laying on the ground in
front of her and lightly whacked the colt's back legs before he ever got
his feet back down on the ground. The colt likely never realized
she was the one who did it but was sure it was caused by kicking in the
proximity of a human. The horse never attempted to kick at anyone again.
Bob