Sounds like upper respiratory but did u try to look down his throat to see if any worms? Those require a completely diff treatment Is it possible to get him a heat lamp? You can very gently suction extra mucus/ fluid hopefully he's a tough guy
Did you just have a change of weather? That can bring on problems, make sure he's somewhere out of drafts but not stuffy needs fresh air , I would compare symptoms to gape worm also ,Hope he feels better soon!