quote:
he's only 3 weeks old.
I hope you really meant to say that you have only had him for 3 weeks.
A 3 week old joey should not be separated from its parents - and should still be nursing from mom.
It this joey is indeed only 3 weeks OOP he may not survive on apples only. He may not be drinking from a dish or a waterbottle because he is too immature and does not know how.
Gliders learn to eat solid foods from their parents as they are gradually weaned. Most do not start going to the food dish with the parents until they are about 5 weeks OOP - which may be why your joey is not eating well.
What types of protein are you offering?
It would be best to choose one of the widely used glider staples that provide a nectar like staple that provided the protein, vitamins and calcium your glider must have to remain healthy. This will also provide a good bit of the liquids he requires. These staples are fed with a wide variety of fruits and vegetables. I have information on my web page for my own GliderKids Feeding plan and links to other widely used glider diets to help you choose one for your joey.
http://www.gliderkids-diet.com/Staple-Recipes.html
However - if your joey is only 3 weeks OOP you might want to see if the individual you purchased the joey from will try to put it back with its parents (assuming they will accept him back.)
It is very possible your joey will NOT survive unless you take steps to make sure he is getting adequate nutrients and enough fluids.