As far as I can tell, he's just testing you to see if he can get away with doing nothing

If he's not in pain anywhere (back, teeth etc.) and he still bucks, he's just being a swine

I had this same problem with my youngster - you basically have to teach them the aids

To get him to canter, kick him once. If he ignores you, kick him again, harder. If he still ignores you, spank him once behind the girth HARD, but be ready for his reaction

He'll soon that if he doesn't move off the leg, then he'll get a good spanking

This was the method I was taught at Pony Club, and it really works XD