a squirrel, grey, about 3-4 days old.
its too big to be a mouse or rat pup but at that age they all look very similar, a mouse pup would be about 1/2 the length of your thumb and a rat pup about 2/3rds, squirrel pups are about thumb lenght.
hes also too "round" to be a ratty pup, they tend to be slimmer, whereas squirrel babies have a much more rounded apearance in body and face.
unfortunatly squirrels under a week old are notoriously difficult to raise even for experienced rehabbers (its also very likley theres at least 1-2 more up there)
KMR (no human milk, cow milk is not easily digeastable, especially after pasturization. raw goats milk will work too but Kitten milk replacer (or KMR) is the best bet. (and probbaly easier to find than raw goats milk unles syou have a goat in milk) you can also find lots of recipies for kitten milk replacer online.
he will need to be kept warm, get him on a heat pad right away.
honestly though, given 1: the difficulty in raising baby squirrels and 2: the legal repercussions (true its unlikely there gonna come knocking on your door for 1 squirell..but....) id find a local rehabber to try and take this baby in.
i have a friend that does squirrels and has been raising squirrels and rabbits for over 20 yrs now as a state licensed rehabber, and even she says under 7 days old squirrels are very touch and go
personally i do muhc bigger animals lol, state lisenced for vector species so im nto as familiar with squirrels...but ive also raised many a litter of rat babies and can tell you its definatly not a rat or mouse lol
