A couple more to put on your list:
Anatomeq makes a stable blanket that is closed front, so no irritating buckles - maybe he would be comfortable in that? I know a few people who have them and seem to like them, although they are pricey.
Horsewear Ireland makes a Dry Liner which is wicking and breathable. I don’t think it has belly straps as it’s supposed to be used along with a turnout, but you could put a no-fill sheet on top:
https://shop.horseware.com/product/horseware-dry-liner-50g-lite/