my mare does the same thing. sit up tall and sit deep and when you ask for the change, squeeze on the inside rein and pull up slightly like a little half halt. this will also help to make him pick himself up instead of dropping his shoulder.

Figure eights around barrels, make him take it slow. Joggle his mouth a bit to slow him down as release when he slows.

My gelding does that also, I have him go over a ground pole, go two strides, change, go another two strides to another ground pole.