lengthening and shortening within the gaits, progressive transitions, and non progressive downward transitions, gymnastics, ground poles and jumps with groundpoles before them, and tons of HALF HALTS!!!!
Gymnastics and ground poles are probably best!
I suggest doing a lot of transitions from canter to walk in your flat work before you start jumping as well as lengthening and shortening their stride. After the first couple jumps try halting if they get too strong. You can also play around with the bit a little. And you can do pole work with halts inbetween a line.
My young horse used to rush so
I did/do lots of grid work and bounces
After a good warm up jumping small jumps in circle!
When I say vee poles it's two poles making an upside down "v" form in the jump. Hope that helps!
Vee poles helps slow down rushing horses.
Gymnastics or ground poles before the jumps. Something to make him think and back off