My friends horse does the same when he’s given alfalfa hay or a high mixed hay. He does great on straight grass hay and out in pasture. The probiotics didn’t help him much either.
I would try changing him to a complete grain(triple crown has a good one).
Have you tried finding a crop that isn’t fertilized with chicken poop?
Maybe that’s his issue?
Does he have the same reaction if he gets the cubes or pellets?
"Take the time it takes, so that it takes less time."
"Expect a lot, accept a little, reward often."