The skin sac you're describing sounds like brumation... But all your husbandry is pretty spot-on so I don't know why he'd be doing it. The good news is it does sound like brumation rather than an illness so you don't have to immediately worry.
How long have you had this frog, and has it always done this?
Someone more experienced in breeding may be able to recommend a way to bring him out by simulating the beginning of the breeding season, but since your temps/humidity are already where they should be I can't think of any other way to bring him out.