Another thought is Selenium deficiency. This will affect his muscles and ability to use them. Can appear horse “just poops out” in work. Selenium is a separate test from what a blood work screen will show.
You may think he is getting enough with mineral blocks, vitamin supplements, when he is not. Only testing will tell you. We bought a horse home to Michigan from Virginia. We were told he had been hunting weekly, very fit! Sore bodied when groomed. He could not keep up with our horses who were only moderately fit! We always test new horses for Selenium and his test returned with him in the danger zone deficient! Doc came out with results and Selenium shots to keep horse from trying to die in work!! Horse was fine, no soreness, plenty of endurance once his Selenium numbers got up into the “normal range.”
We supplement all the horses with individual dosing of the Selenium and Vit E mixed. Vit E must be fed because it is needed to help absorb the Selenium. This is 1/8 teaspoon per horse, per day, to make sure they get enough. We have no Selenium in our local soil, so supplementing is required. Working a horse so it sweats often uses up Selenium, making supplementing even more important to the horse.