You can try feeding organic soy infant formula or Kitten Milk replacer, thickened with a bit of baby cereal (the kind that says add milk)
Grocery stores may sell the formula, if not then you can order it from Walmart. Kitten Milk Replacer can be purchased from a vet
He may eat it out of a shallow dish, or lick it off your finger, otherwise you can give him a bit into the side of his mouth one drop at a time so that it does not go into his lungs
He may be dehydrated (dehydration kills), not getting proper nutrition or he may be ill
Try the formula and also make a vet appointment with a good vet with the knowledge and experience to treat rats
Is his stomach bloated?
Is he pooping and peeing normally?
How is his breathing? Laboured? A lot of effort?
Please post pictures
and post videos - post them on youtube and then put the link on here
Do they have a water dish that can not be upset?