There are things that can be done to help when they only have a touch of diarrhea with no other real symptoms. However when you notice that he isn't acting right it's time to get him to a vet. Gliders, by nature, will hide an illness as long as they can...so by the time you see symptoms they've usually already been dealing with it for a while.
You'll need to make sure he stays hydrated...offer juicy fruits like watermelon.
Let us know if you have a good exotic vet who can see him asap. If you don't, post here for information on vets that may be in your area and more advice on things you might be able to do in the mean time.
I don't want to offer a 'bandaid' when it seems clear that he needs to be seen by a professional.
We know how you feel...sorry he's not feeling well.