My riding teacher has a horse like that, except it will run of with you if you don't keep its head down. We invented the Blue Thing. It's a small blue rope with clips on both ends that we attach to the saddle and run through his jaw strap. Works wonders!
Circle right before the jump. Just do random things to keep him on his toes! Some horses start doing that when they anticipate a jump
running martin gale and working on getting him round in flat work helps for me
Do lots of flat work and you can also use draw reins in your flat work to get muscle in his top line. Also, you can try a running marti
Some horses just find it easier to jump a jump with there head up helps him through his body over the jump (in a way).
Do more flat work. You. Need to become more supple through the back. Try more honches in or shoulder In. Do some counter canter. Because once the horse becomes truly round on the flat. The rest comes .💖
NEVER try to cover up issues with a gadget. they may mask the problem for a while but in the end the problem gets worse. go back to softening and suppleness. work on lots of stopping, direction changes, and transitions