You need to start introducing him to other foods as well as his formula. When the mother is resting the babies get off to forage and explore and get back on when mom is ready to get going.
Start with a little yogurt. Fruit flavored yogurt is OK.
All fruit needs to be cut up on introduction. Grapes, any kind of berry, apples, pears, plums - just about anything except citrus.
mealworms, crickets, fish, dare I say it - chicken and eggs
kale, brocolli, green beans, cauliflower, squash, sweet potato, etc
I have a pet possum named Golum. She is very sweet. I kept her because she was very ill when small and she was handled too much and won't run from anything.