Halt immediately after the jump! Also can try grid work... but still end with a halt!
Do circles after the jump until he slows down
Once your over the jump stop him as soon as you can then pick back up again at your own pace. Do this a couple of times and eventually he will understand to slow down. Hope this helps!!!!!
Try halting after a jump and doing lots of gymnastics. Poles after the jump also help!!
Poles after maybe before if you think it will help to keep him thinking about his feet. Not only when jumping but you can use poles when flatting also to keep him on his toes and using his brain