I know it has been mentioned quite a bit, but saddle fit is the first thing I would check as well. Do you ride western or english? Though you may think the tack is fitted correctly, I would consider having a professional check. (Some tack shops offer this service).
If this is strictly attitude, I would treat his kicking out more seriously. Kicking out once in a while has led to kicking out every time, which may lead to more problems if it isn't corrected. My gelding had a similar problem, but it was due to a previous rider allowing him to get away with bucking. Ask him to canter and don't act like he will kick out, at least give him the opportunity to behave. As soon as he kicks out turn in a small circle with one rein, and give him a couple really good smacks on the butt with a dressage whip. He needs to know what he did is wrong and that you won't tolerate it. Then ask for the canter again right away, and if he kicks out repeat the same thing again. Do this until he goes into the canter without kicking out. This is how I corrected the problem with my gelding, and his bucking was corrected quickly.