YOU are the one that has to get your strides right, not the horse. Use half halts to slow/collect between jumps if he's rushing. Sit deeper in the saddle between jumps, encouraging him to slow and relax. Canter poles between jumps can help to, to force him to slow and think.
If he's chipping or adding strides, then use a half seat between jumps and really push him with your leg to extend. Use light half halts to keep him under control, so he doesn't learn to rush.
Lastly, get the help of a trainer