If you really want to help this horse, Re-examine your beliefs of what a horse ‘likes’. If you have truly ruled out physical limitations, then it is misunderstanding between you two. FIRST, You have to be willing to change before he will. Second, Horses are lazy by nature, is he working FOR you or FROM you ? Third, Education trumps gadgets every time, it’s not what is or is not in his mouth, or what you are sitting on/in.
Please enlist the guidance of someone who no longer has a ‘hot’ horse and not just take advise from those that accept that is just how their horse ‘is’.
Keep it simple, but don’t expect it to be easy.