I guess that I never really appreciated that all of the hours my old herd spent in lessons
developed their muscles to the point that when I asked them to perform movements, it never hurt. Dressage movements, INCLUDING basic collection, takes a LOT more muscle than you realize.
Horses develop a buildup of lactic acid just like we do, and they don't like to work if it causes any pain.
I suggest that you also cross train. I'll bet you can find some trails or terrain with hills and work him on a loose rein while he has to ascend and descend, developing his HQ's, and giving him something interesting to look at.
I also like downward transitions between gaits using half halts. He will feel them coming and use his head and neck to help slow down. Be very gentle with the bit. I'll bet you can find more than 3 speeds/tempi at the trot and the trot is the most balanced gait to use for training. I like to push into a working trot after I half halt to a walk, then repeat.
You can also find several speeds/tempi in the walk, ESPECIALLY the extended walk, that speed where he almost wants to break into the trot,
but doesn't. This creates a LOT of swinging motion and that keeps your horse limber.
Remember to use a long, relaxed, medium walk to warm up AND to cool down, in case you don't already do this.