I would change your grid so the distances get longer instead of shorter. So I would go maybe Xrail-Bounce-Xrail-1 Stride-Xrail-2 strides-Vertical or a variation. If he is rushing into the grid, the bounce will make him sit up and shorten himself (make it a short bounce) You say he rushes, how is he at shortening his stride? Is he easy to cellect into a short, bouncy canter? My last horse who rushed I would redirect his forward energy into upwards energey, creating a hugely impulsive, bouncy canter and he would then take a short spot over a long spot, which made him really bascule over the jump and jump up instead of flat. Also, don't be afraid to circle/halty in front of jump until he is at YOUR speed. It won't teach him to refuse, as you are asking for the halt/circle. Lots of circles on the approach to the jump work well, he will rush when facing the jump but being turned away every time he will settle, once he is settled take him over the jump.
Another good exercise is setting up jump in a line, end to end, kind of like this: - - - - But with a fair distance in between. Ride them like a serpentine, doing a half circle to reach the next jump. Ask for a simple change in the middle of your half circle, or if he is already on the correct lead, a few steps of trot before picking up your canter again. Because it is fairly tight, he will have to think about what he is doing, it isn’t a straight line so he can’t see or anticipate the next jump, and the simple change/trot strides are making him pause.
You guys are a great pair, and boy is he cute!