Groundwork, groundwork, groundwork. Jump him from the ground at various heights while on a long rope until you see he starts having more rhythm and calming down.
Another thing you can do is ride slowly towards the jump and stop him in front.
Chances are if he's rushing he's afraid, stopping in front and taking a break will reassure him.
Ground poles on either side of a fence help to slow them down. Doing a lot of gymnastics also helps them to think and slow down between the fences.
You have to careful using this one (as if you're not it could teach them to refuse) but you can ask them to halt as they're approaching the jump as soon as they start to rush. Another thing I've done with my boys was use V-poles (look it up! So useful). Really makes them think and have to be careful, every horse I've tried it with immediately slows down.
I had a tbred once that rushed fences and had a limited stride. Everyone seemed to try and get me to trot fences hold him back etc nothing worked . He would bear down a run at the fence. I was In a Greg best clinic. He had me lengthen my reins
I agree with everything said so far. Also, an exercise that I find works is having a crossrail on a circle, and just jumping it over and over again at the trot until the horse gets bored and stops rushing. Then do it at the canter. This worked very well for me with a pony I worked with last year who rushed to everything and couldn't keep a consistent rhythm in general. It takes time, but aside from stopping in front of the jump, it's all I've heard of they helps.
I agree with Tessy completely. Going slowly may actually increase the horse's rushing, as the horse may feel it does not have the power to get itself over the jumps! :)
I'd take a page out of the hunter training book here, and not allow the horse to jump unless it is quiet, not pulling, and not rushing. As soon as the horse starts to pull/rush towards the fence, quietly halt, or quietly circle away. It needs to stay on the same rhythm all the way to the fence, without yanking your arms out to make it happen.