Lots of trotting especially up hills and poles
The first thing to do is to make your horse regain muscles and cardio. To do so, I recommend rides out with hill work. If you can't go out, I agree with Amber saying to work with poles.
Make sure he is in good shape before starting to jump again. Start with small jumps and simple exercises. Gymnastic lines are very efficient.
Good luck :)
Start with basic ground work then start walking and trotting poles and circles and keep ur horse on the rail at a walk then trot then canter