This happens with my elder geldings every year. The hay, no matter how fine, is still coarser and more fibrous than fresh grass. It’s also drier. It’s not the protein that’s causing the problem. My guys also react when they switch back to pasture, but for a shorter period of time. You might try switching his regular grain ration to a complete feed that’s higher in fiber so the change to hay isn’t so dramatic for his system. Make sure he’s well hydrated, even if it means soaking his feed a bit during that time period. Otherwise, I doubt there’s much you can do for this poor sensitive guy. But your vet might have suggestions. Give him/her a call and see what the recommendation is.
Horses In the Yard