Look's like you got a red ear slider FOX. My sister got 2 last year for my nephew and somehow they got dumped on me.
They were that size when she got them.
FOX's picA year later and 2 aquarium's later I have these 2 eating machine's. Pics are a few months old.
Just wash your hands after handling the little fella and give him a lot more room to swim in and get a good water filter and a good lamp for basking.
He still may be to small but a few small guppies are some of that dried shrimp they sell at the pet store(grind it a little so the little guy can eat it) and those little pellet's are all you need.
They grow quick(real quick) and are a commitment. I enjoy watching my 2 idiots try to catch there meals(guppies,gold fish,rosey red's)and theyre easier to take care of than the dog.
Oh yeah when it gets a little bigger there's a powder that you put in a container and put the turtle in for a few hours to clean the little bugger and prevent sickness.