I would get a blood test and/or fecal exam done.
waiting until the appetite is lost could be a bad idea. Which I think you understand.
7 years is on the more aged side of a Glider Lifespan (you must be a fabulous suggie parent!!)
keep loving on him, keep on getting him to eat. Has he lost any weight? does he pee often?
the only things I can think of right now with this would be some sort of parasite or internal disease. I know how expensive those vet bills can get. You know what's best for you and your Gliders.