@annawatkins as i said in other answer about crop and spurs. It' difficult to stop him and get him collected but not to stop moving.
About health problems. We already checked him, he doesn't have any issues.
My horse did that as well. I started using a bit less bridle so she felt she wasn't being held back. I now don't need to use it and she jumps perfectly! I would also test for low Vitamin E levels and maybe ulcers.
I agree with the other answers, it might just be that he isn't jumping any higher than that now. Also, you should check if anything is wrong with him physically. Otherwise I would just sit back add leg and move him up to the jump. You might have to start using a crop to smack him right before it.
When you are jumping him if he refuses you can stand in front of the jump and show him that it is fine.If that doesn't work then maybe contact your local vet or farrier to see if there is something that is bothering him.Hopefully this helped!