So sorry for what you're going through. I wish I had some other suggestions, but other than what you're already doing, I would strongly suggest trying to find someone who is familiar with sugar gliders so you can get your little one checked out. It's hard to say what could be causing this, as well as the appropriate treatment. A qualified exotic vet would be your best bet for making sure your glider is getting the best care. In the meantime, I would keep a close eye on him, keep him warm, watch for signs of dehydration, and maybe try offering some favorite treats that he might be more likely to eat (even apple juice or orange Gatorade might be more appealing to him and ensure that he's getting fluids. Hopefully someone with more experience here will reply and have some additional ideas. Wishing you both well!