I, too, was going to recommend a magnesium supp.
Have you had him checked out by the vet? when was the last time his teeth were done?
Also, what kind of feed is he on? any grain? is he on a low carb hay? he may have some metabolic issues, just a thought. If you can, I’d try switching to a low carb grass hay, get him checked out by the vet, do the mag supp, and reevaluate in several months.