I’m not saying don’t do this, but I will say that if skipping a few treatments would allow for blood/hair tests for muscle issues, I personally would be skipping treatments and getting some tests run. My other concern would be feet and guts, although I’m pretty sure you said you were sure it wasn’t guts in your first post.
Barring all that, give the guy a holiday from training by hacking, playing with obstacles, jumping tiny jumps, even schooling on different footing in a different area of the farm (if there are grass fields that are ever available, LOTS of horses actually prefer grass footing :o ) ride out there for a few days, longe, long rein/ground drive, work on something that you’d never work on for competition (Spanish walk for instance), even hand walking on a trail. Anything that will allow you to both develop a better connection to each other, a bigger trust, on neutral ground.
If the horse is safe and trust worthy, hop on one day and drop the reins to the buckle, ask for a walk and see where he takes you. That game is one of my personal favourites with my current horse. I would never have known she was so curious, adventurous and such an eagle eye about all the things in “her” farm if she hadn’t become sick enough to not be able to work, but not sick enough that she couldn’t carry me around on little walking adventures. If your horse automatically starts out for the arena, help out a little by changing the direction and then dropping the reins to the buckle again and again if necessary. Some can take a while to realize that this is a special time they get to take you for an adventure rather than being ridden.
Editing to add that once you get to the point of enjoying schooling/training, be sure to continue a couple of the things you and he found most fun at least a couple of times a week. I promise your training will not suffer, in fact, you may find it improves.