Half halts and downward transitions whether it's within the gate or downward into another. Be sure while doing this however to use your upper body and legs not the reins.
Cavallettis, in and out jumps, backing up and alternating between stretching the neck down and forward and raising the head having the poll as the highest point.
Well if he rushes on the front end he's probably loading more weight on it so the focus will be relaxation and balance.
Very frequent downward transitions (by that I mean 4 transitions in a single 20m circle), cavallettis
Half halting every few steps, this will make sure they stay sitting up on their hind end and don't just rush forward. Also keeping a good hold on the outside rein to prevent them from being able to rush out the front.