Pie seems to be completely back to normal. I visited him in the pasture this morning after riding Dawn, and he was chowing down on the grass and was perfectly comfortable. His stall wasn't torn up, which meant he'd been quiet in the night, he'd finished all his night hay and eaten his breakfast, and there was plenty of manure. While I was checking on him in the pasture, he considerately passed a nice pile of completely normal manure. And this afternoon, we had a lovely ride in the pasture - it had been very cold and windy this morning but the sun was out and it was a beautiful afternoon.
I expect it was the weather change that triggered his discomfort, making him gassy. Fortunately, he's a big drinker and I never have to worry about him getting dehydrated and having an impaction. Maisie was prone to that, as she was a poor drinker in the winter, even with a heated bucket.
So far, so good, but I'll be keeping a close eye on him to be sure all is well.