(Im not a trainer ! I hope this helps!I️)Try to slow him down with your leg more then your hand, if you have to practice this at a trot then do so. When you do canter and/or gallop, (i’d do this at home too) every time you slow him down focus on it coming from your leg clenching him to slow down first then your hand following up to slow him down. Make it feel gradual. I know that gradual stops sounds terrifying to think about when you’re on a bolting horse, but try your best with it.
When training X country, choose a jump and get him to halt a few strides after it. This makes them think and listen to your hands. Do this on a few different jumps or even after a line of X country jumps. It's about getting them more responsive to your hands and if you just try to slow him down all the time he will stop listening to them xx