Some suggestions are: avocado, peanut butter spread thinly on a cracker, oatmeal, eggs (hardboiled and mashed), baby food (cereals, fruits, and vanilla custard), bananas, and cooked pasta.
If he is too sick to care about food you will need to hand-feed him. You will need to have some large syringes (no needles) for this. Good sizes to use for feeding are 3cc, 6cc, and 12cc. It is too awkward trying to feed with the large syringe; that is why this method is preferable. Try having several different flavors available at one time, so if your pet gets bored with one, you can move on to another and recapture his interest. Keep a rag or tissues handy as hand-feeding can be messy.
Severely ill mice or rats that need to be hand-fed will also need to be given water by hand. Do not put a bowl of water in your mice or rats cage because he will tip it over and get himself wet possibly making his illness worse. Instead, mix a teaspoon of sugar into 4 ounces of water and administer as much as he will drink with a small syringe, as often as you can give it to him.