Lots of half halts. Start with putting poles leading up to a small jump, and stop him about 4 strides away from the jump if he went too fast. Best of luck :)
I would practice doing half halts around the curve. You can start cantering over pole and around the curve do a half halt to make your horse listen. Hope this helps answer your question!
I use poles on the ground. When you start jumping again place a few trot or canter poles in front of a small cross jump. You just have to do it a few times a week rest is flat work.
Try to slow down your pace all together. Repetition isn’t the key. The more you jump, the greater the break down of your horses joints over all are, which is leads to a painful and short future. Start with small poles and work your way up but try just at a trot for a good amount of time before moving up and increasing needed momentum. Good luck!