The way you describe him sounds just like my mare when I got her at 5. Nothing done. Have had to restart her a couple of times. I think she has worse conformation although at 7, she is starting to balance out. I have the same issues as you and am learning with her and making slow, but steady progress. Same aim as you too
very nice horse, you should be able to do lots of things with him in the future.
My advice would be to get him into sidereins asap, lunging and get on his back walk, trot, canter and leg yielding as soon as possible. He is behind the curve in the MENTAL adjustment needed to do more than trail ride and loaf around