Eh good question 😂 I'd say keep going with the trotting poles and raising one end and then the other end. Both in-hand and on him. Then try raised trotting poles in the same way. It could help to have someone on him and hand him follow you from the ground and then on him? It might take some time but with lots of trust and praise he'll get there!
He will follow me over and lunge over. I had his tack checked and even tried changing bits. Even tried going over bareback with just a halter and same response. He either gets up to it and stops or gets about 2 strides out and will start backing away. He always used to be the horse you could turn out and he sometimes go over the jumps himself. I have started back to basics working on the ground and flat work and over ground poles.
If he has been cleared by vets and you're confident you don't need another opinion then either he's done a lot of jumping and turned sour or his reaction stems from fear. Beating him over it isn't the way I'd go about it if there's a genuine reason. Maybe take a break from jumping and work on your flatwork and your relationship? Does he loose jump or is it only when a rider is on? Has his tack been checked? It could be pinching? Especially if he's growing or gaining muscle his body shape will change and so will how his tack fits. I don't know if you're info horsemanship or not but you could try exercises like join up and see if he'd follow you over raised poles or small jumps from the ground?
My horse was the same way. But every time she did that, I turned her face to my knee and smacked her with a crop 2-3 times or kicked if I didn't have one. Then I kicked and hit over it and repeated until she did it. She's fine now and only runs out or stops when she is scared. Also if he starts to stop kick and smack to make him go faster. Be careful though, it's easy to fall of if your too wobbly so keep your legs on.
He may be anxious about somebody catching him in the mouth (not releasing over the jump or landing and imidiatelt pulling on his face) try baby steps with him and see if he reacts differently if you overrreact your release on him
Has anyone fallen off him at a jump recently? Maybe he's just being sensitive and worrying about his human lol. Try taking him up to it just leading h by the reins. Start with small things so the you can jump them with him from the ground.