do lots of downward transitions to force him on his hind. ride with higher hands to "lift" him. do lots of circles and shoulder outs
You saddle might not fit. I'd get a saddle fitter out
I would canter, and if your horse gets heavy on your hands, stop and back them until they are soft. Then, canter off again, and if they are heavy, repeat backing. If they are light for like 5 strides, walk and make it peaceful. Keep them light for longer and longer until it is normal. Hope this helps!
Try only cantering him when he is in frame even if it is only for a few strides. As soon as he goes out of frame ask for a trot and do it over again.
Try doing different exterises he might not respect you