Best of luck with your horse!
Mister had an impaction last fall, the same day the remnants of Hurricane Hannah swept through. The vet thought that the weather set it off, but I think it was the round bale I fed out. Whatever it was, I noticed Mister trying to poop, but it would actually suck back inside him. He was miserable. I called my horse knowledgeable friend, who came right out and said it looked like sand colic or an impaction. The vet arrived and confirmed an impaction. She tubed him, gave him some banamine, and within an hour he was pooping again - thank god! (It was about six hours between seeing something off and the vet getting there.) The vet recommended giving him a shot of mineral oil in his mash for a few days to be on the safe side, and some gatorade to keep his electrolytes up that day.