If he is sound, just be patient and keep trying. Make sure to reward him when he gets it right. Lunging him can help too.
Have him checked for ulcers. Also have a chiropractor look at him. If there is nothing, when you canter him dont let him drop down to a trot when he kicks or bucks. Also lunge him and get him to move when you are asking him to canter
May be a back problem but try backing him up 3-5 steps when he does it to show him it is not good
Get him checked out. Might be a reaction to pain. Once that is ruled out keep your leg on him. Maybe make a small canter circle. Then back to the trot and try the transition again. Just don't give him release when he kicks out.
He doesn't do anything afterwards ,and I can stick it so I don't think he is doing to bad. I don't know if he doesn't know it is not ok or what