Chronic diarrhea in 27 year old horse

My Appaloosa gelding has been getting full on projectile diarrhea when eating hay. The horses are on pasture board but when the footing in the field is wet, they get hay and are confined to a dry lot. About 24 hours after getting hay, he gets diarrhea. The diarrhea resolves after returning to grass or with biosponge treatment. This has been going on and off for maybe 2 months.

He's been examined by a vet. Physical exam is normal. Blood work is normal. He doesn't seem to have trouble chewing and his teeth are fine. He has all of them. None are broken or loose and he is up to date on floats. The vet was unable to feel any points or other issues. He has no fever, and no loss of appetite. His attitude is bright. The other 2 horses in his field are fine. He was just dewormed and his fecal exam was negative. He's in good weight. He is being treated for Cushing's with a daily half dose of prascend and he's on probiotics.

The hay is a high quality grass hay, the same kind of hay he's always had, and none of the 18 other horses on the property seem to be having an issue with it.

There has been no change to his turnout situation or his workload (been retired for 2 years). He hasn't left the property. Other than the grass to hay switch, his diet remains the same.

I read that chronic diarrhea can be a common issue for older horses. Does anyone else have experience with this? What else can I do to combat this?