How can I slow and control my horse when jumping?

Get him focused by collecting his head down. The more collected, the more focused. Make sure you aren't getting excited either when you approach the jump, horses can sense your energy. Also, try slowing him with your SEAT as well as your reins. If you slow your body down, your horse will follow. Horses like when your bodies are moving as one, on beat. So if you slow your whole body down, it will force him to slow down. Work on collection and controlling your seat and you should be good. Hope this helps!