Too slow into the canter, popping outside shoulder

whoooo.... wow. that's a scientific title, ain't it???

Chewie's picking up a habit.... and it's why I didn't do a second flat class at the show. Asking for canter to the left, he's great. Sit back, squeeze, kiss... no hard work, if he's on a corner, he'll catch his leads, and be quite cooperative. Stays in it, but only , only , ONLY , if I keep my leg on him. He's accepting contact, provided I move with him. I'm starting to learn how to "wrap my calves around him at canter", keeping him moving. If I stop working at it for even a second, he'll break to trot. Any tips? I don't think this is entirely bad, but a little concerned that if I go to half-seat for hunters, it'll be nearly impossible to keep him at canter.

To the right, it's a ball-game. He hops up into canter, and that's not so bad, but it's tossing off my balance. That is just going to take practice remembering to lean back ( I can still hear my trainer telling me "you've got to lean back, or it's just going to get worse ). After he's cantering, if I don't have a ring "wall" to keep him together, he pops his outside shoulder out, and turns only his neck, keeping his back end going wherever it was. I was trying to keep the outside rein tight, putting up "the wall" for him, but it sure felt much MUCH tighter than the inside rein. I know I should "ride him through it", but I'm looking for some tips on the easiest way to keep him from trying it. In the class, as I asked for canter to the right, he popped UP (big, I thought), started to throw his head, and I finally got frustrated with him, kicked him (thunk on his sides), and he immediately corrected. By time the judge was looking, trainer said I looked great, and so did Chewie. It felt horrible. I figured he was on the wrong lead, and I didn't want to look, I simply wanted him to ride through it & be done. I thought to self "if it's the wrong lead, maybe he'll at least learn he can't be a snot when I ask." Trainer said it probably felt bad because when he went into it, he sunk his back end down, hopped up in front, and when I kicked, he probably lost just enough momentum in his temper tantrum that he got confused enough to forget what he was doing, so he was still trying to put it back together by time the canter was over.

So what's the trick? :) Okay, I know there won't be a trick, but what's a good way to get him over it? I know it's primarily him testing me. He's trying it, because it's usually working. He'll be good sometimes, then it seems like every once in a while, he'll try to avoid my cues, and give a little head toss, trying to get out of work. Overall, he's slow, and while I should be complaining about that, I'm not. I will work on the speed, later. Once he's a little more put together & respectful, we'll work on going faster.

Until then, I'm going to stick with pre-beginner & beginner work, thwapping egos of little kids & teenagers on super-pricey show horses. :) WAHoOOO!!