Well, if it is a rodent, feed it puppy or kitten milk replacement. I had a baby bat for a week (long story) and I fed him every 1 hour and 45 minutes.

A heating pad is not the best. It this baby finds the right spot, it could fry. Try placing hot water bottles under a towel around him. He will get close to those, and snuggle in.

Honestly, I think it is a squirrel. The size is right and everything. I can research tonight (right now..) for you, and try to find something, and figure out if it is a squirrel.

Honestly, a vet isn't the best place to bring it. At least from my experience. They told me that my bat would die that night, and that he would give everyone rabies. They refused to give me any information. Okay, yes they aren't the people I typically go to, but it was an emergency. I called my vets from another town, and they had no clue on what to do.

EDIT: The formula brand I would suggest using is Just Born, puppy formula is what I would go with

Good luck, keep us updated!
Steph