A little more information would help.
What are you feeding him each night?
Is he "always sleepy" and moving slow during the day or is he also moving slowly at night after the room is dark?
Gliders are nocturnal animals and you are not going to see much activity during the day time. My gliders will not come out of their sleeping pouches until all the lights are turned off in their room.
If you have the lights on in the evening, try turning them off and see if he is more active in the dark.
If you have not already done so, a well check up with a glider experienced vet would be a good idea to make sure he does not have any health issues that would contribute to his decreased activity.
Another thing to consider is that gliders are colony animals and do not thrive when kept alone. He may be depressed due to being separated from his parents and not having another glider companion. I hope you will consider getting him a cage mate soon.