I don't know if you want suggestions, but I might ease off on the rollbacks and work on walk, trot, canter transitions and not just the transitions but moving him up and down with the life in your body. For ecample at a trot see if you can get him to trot as slow as possible by slowing down your posting, even getting a little behind the motion to bring him back to meet you. Then post faster and see if you can move him right up to a long trot and even right into the canter. Start posting in your canter and he should drop back to a trot for you. Most horses will think you are bonkers and shift their rhythm to meet yours. The benefit of this is that he will be much more in tune to your feel and you will not need to worry about rein and leg aids. Pretty soon you can sit up ready to go and get a nice gallop depart from a standstill if you want it

"It's really quite amazing what a horse will do for you, if he only understands what you want. 
And it it's also quite amazing what a horse will do to you if he doesn't." ~ Bill Dorrance