I agree with Overo Mare, sliders are very tricky pets! They are time consuming and a mess, and will need a very large tank once they are older, as they grow BIG.
That being said, I had a red eared slider for around 10 years, had him since he was about the size of yours. As much of a hassle he was, he was one of my favorite pets! They are incredibly inquisitive, and just so fun to watch!
If you plan to keep adorable little Tilly, I would start him in a 20gal long type tank. At this age, it is best to have the water portion of the tank just deep enough that they can touch the bottom with their back feet, and stick their heads up to breathe. This takes away any risk of accidental drownings in a deep tank (unusual, but does happen when they get way too tired). You will also need a float dock (sold at pet stores) or build a partial area of dry land on one side of the tank. He will need a place to get out of the water under a heat lamp and dry off.
Biggest thing I tell people who want an aquatic turtle is FILTRATION. Get the best filters you can afford, as these guys POOP a LOT. Also get bigger than the rated size is. If the filter says its for a 20 gal, go with the next one up for a 30 gallon. You will need to keep up on the cleaning in his tank. Some people keep it bare (does make cleaning a breeze) but I've never liked the look of a bare tank, so large pebbles are fine too
As far as feeding, I would do a base feed of aquatic turtle pellets, and supplement with fish, worms, krill, etc. They will really eat anything they can fit in their mouths, so it is fun to get them treats of random things. Mine absolutely loved cave crickets lol. Feeding fish is also fun, cause some won't get eaten and then you have cute little fish to watch as well!
I would get feeder minnows from the pet store (and the pond, depending on the time of year). I also had luck keeping a large pleco (bottomfeeder catfish) in the tank with him for like 7 years. I loved that fish, lol. I also kept a couple large goldfish with him, but the goldfish will eventually get killed, so don't get too attached!
As far as sexing him goes, you really can't tell externally if he is a male or a female. I think it was maybe a year? Maybe a little more before I could tell what gender mine was. Mine turned out to be a male, and got to be around 10 inches long within the 10 ish year time frame.
Anyway, whatever you decide to do with Tilly, good luck! He is an adorable little dude!
