What is wrong with him What should I do

I have a 16.3 hand Appendix gelding that is around 14 or 15 years old. I got him when he was 13 and he was an awesome english pleasure horse. He used a fast/tight twist bit and was very responsive to it. I started jumping him and he loved it. We went to a show and won all of our classes. He rarely bucked and his head used to be so low and he was anlways focoused on me. I started taking more jumping lessons and he started getting out of control. Now I retired him to trail and flatwork. He still acts up and he doesn't act like he used to.. What is wrong with him and how should I resolve it?

I too am going to guess that this is caused from pain. Whether it is being caused from the rider over the jumps or prior/after the jump (like being on the face and constantly bumping the horses mouth at a certain spot) People that are poorly balanced tend to use the reins as a brace instead of their legs/seat and this can 'cause behavioral issues in itself and get that horse "unbalanced" while trying to do his job. Or, ill fitted tack can also cause these kind of behavorial issues. However, it very well could be a physical problem with the horse. As stated, his spine or a rib could be out and these things are not always picked up by a vet, unless they have studied alternative or "eastern" methods. Teeth, stifle/hock injuries, arthritis and all other sorts of problems could be the underlying issue. And ofcourse, overworking something or doing training too fast can also lead to these sorts of issues. At this point you have to start "ruling out" all of the potential problems. Check your tack, have professionals both in the vetrinarian and alternative feilds evaluate your horse and also look at the training. was he pushed along to fast?