puppy milk replacer mixed with a little whipped cream is said to be good...first you need to make sure it is being hydrated or able to use the water feeder..
baby squirrel care--from a wildlife website
Orphans that have been without their mother will be suffering from chill and dehydration. They must be thoroughly warmed first, and then, although they are starving, they must be given warmed rehydration solution before any milk formula is offered. Their dehydrated little body is simply unable to digest food (i.e. the milk solids in formula) and if given formula or other food before they are rehydrated it can kill them, or cause debilitating diarrhea. Pedialyte is a rehydration solution made for human babies, and is available in drug stores – it often comes fruit flavoured, but if you can find the unflavoured kind that is best for wildlife babies. It should be heated to body temperature and offered frequently: every 30 minutes to babies that will take only a small amount, or every 2 hours to those that take a larger amount. Feed only Pedialyte for the first several feedings – as much as the baby wants until it is rehydrated and producing lots of light yellow urine when you stimulate it. Stimulate it at each feeding using light feathery strokes.
In an emergency, a homemade rehydration solution can be made by mixing: ½ teaspoon salt + ½ tablespoon of sugar + 2 cups of water – warm slightly to dissolve sugar and salt. Use this homemade solution only until you can get to a drug store. Pedialyte is a balanced electrolyte solution, much better for the baby. Once Pedialyte is open refrigerate between feedings, and discard any unused portion after 72 hours. It can be frozen in an ice cube tray and the cubes stored in the freezer for use within a couple of months.