I'd try trotting in two-point to allow the saddle the maximum freedom to move with his back.
When he gets too forward, use a lagging rein. Don't know what its real name is. But take light contact with the bit, and as each shoulder goes forward, lag with the rein. Don't pull back, just don't let that side flow as well forward. I was taught to do that at a walk first, then trot, then canter. With practice, it will tell the horse not to step out so far with his front leg, but not to slow down. Combined with shifting your balance some to the rear, it seems to help them take smaller steps with the front while still moving with the rear. I find it very useful at both the trot and canter with my mare...