Maybe try wear spurs on the flat and if he doesn't listen you are quite harsh because nagging in the long run makes them more lazy.
I agree if the horse gets all the implosion from jumping then swich to flat so it uses the empolsion in to that
Cooperate flat and jumping. Do pole work on flat and add in small jumps. Do lots of riding in between jumping such as 10 mins of flat a couple of jumps and then back to flat for 10 and so on.