Young horses often get frazzled or confused easily, so make sure you're giving the correct aid and give him time to respond. Really think about the aids cause even a little change can cause them to panic. But then again if he's cheeky and thinks he can get away with it, that needs to be sorted quickly!
Every time he bucks spin him either on him or off of him and make sure you spin quickly (it's what mothers do to reprimand their foals) I did this every time a horse I was breaking bucked and after a while he gave up. It's a kind way to teach them to quit bucking without making them shy or non responsive to aids. Be sure to change the spin direction so he doesn't anticipate