There are a lot of things to do to calm down your horse in front of a jump.
First of all, you need to make sure it is not a problem of posterior limb. For that, you can lunge your horse and see how he reacts approaching the jump. If he's still tensed, maybe you should call an osteopath.
If he's not, it means you need to work on that particular point. To do so, you can start by making small jumps on a circle (it will be easier for him to find his place). Do it several times, the first will be complicated but then he will get used to it and calm down even though you don't ask.
Then, you can work on dressage with obstacles all around the manage to make him get used to it, do your session and when you feel he's relaxed, go jump one little obstacle. Work on your transitions and if it still doesn't work just make him stop in front of the jump, then go for it and stop him as soon as you can after the jump. Do it several times, congratulate him when he does great (and calm) and slowly he will understand that it is better for him to jump that way.