I am new to suggies and this website but I will give you my best advise as of this moment. Hopefully you get more opinions from people with more experience but for now, here is what I would suggest or say.
To begin with, you really should get him warm. Put him on you/your skin to keep his body temperature up. You can
help him get warmer by just covering him with another blanket but him being on you would be better and warmer for him. He would be better off having contact with you since humans have a high body temperature.
Also, do you just have him or do you have another glider with him? If you don't have another glider I would really recommend getting another one but that topic is for another time. If he does have a partner you should probably separate him to a smaller hospital cage so you can both watch him easier and make sure he's moving and so if he's sick he won't infect your other suggie.
Next, you said that the only vet nearby you is gone overseas? Well if you don't mind me asking if you say where you live me and others can help you try to find another vet nearby. Also in my opinion, if he is barely eating and is already lethargic you NEED to get him to a vet as soon as possible or most likely it will just get to a point where it would be too late and he could get way worse. If the one closest to you is gone search for another vet nearby/within an hour or something.
He sounds like he needs to see a vet soon because sugar gliders hide their sicknesses very well so by the time they become lethargic or show symptoms that something is wrong it could be close to too late or already too late. I really hope that is not the case but with not personally being around the suggie to see how bad it is I can't say anything for sure.
As for him not eating/drinking, I would say to get a needless syringe and try to mix some Pedialyte with water and give him some through the needless syringe. It would give him some sugar and a little more energy and would keep him hydrated.
Besides what I stated I'm not very sure of any more information or things to help. I'm very sorry you have to go through this with your poor fur baby and I hope he gets better soon. Also please keep in mind some of the things I stated may not be accurate as again I'm very new to suggies and have yet to deal with a lethargic suggie but this is what I would recommend to do. Also in the meantime, you can try to search on the website for posts from others about lethargic suggies to see what responses and tips they got. And again I'm very sorry you and your poor baby have to go through this.