I don't know what your level of experience is, but if you are not an experienced rehabber you have been plunged into the deep end of the pool with a baby this young. This is a link to an excellent guide - it is several pages long and the "next page" arrow is at the top of each page instead of the bottom:
https://www.henryspets.com/1-caring-...baby-squirrel/
Pinkies need frequent feedngs of spcific amounts and need to be fed the proper formula. What are you feeding?
I will guide some pinkie people your way, too.
In the meantime, this is likely a little skin staph infection. From your local drug store, purchase a bottle of Betadyne (10% povidone iodine) in the wound care aisle. It is the orange stuff they use in surgery. Dilute it to 1/4 cup water and 15 drops of Betadyne. Warm that solution up and using a cotton ball, wipe the baby down with the liquid. Wrap him up immediately and get him back on the heat - you don't want to allow a pinky to chill. I would do this twice a day.