NO GRAVEL. They eat it and it can cause death because it fills up their stomach or gets stuck in their intestines.
Make sure to give him an area to get out of the water too, so he can dry off and sit under the warm light. It should be easy to make something out of some larger rocks, like a brick. Or they have the basking docks that you can stick onto the side of the tank with suction cups which work out great for a turtle your size.
I would just get him a minimum of 20 gallon right now so he can be more comfortable & have more room to swim (feel free to fill the tank at least 1/2 way, but make sure that when he gets out of the water to sit on his basking dock that he can't climb out of the tank!). Keep in mind that a little teeny turtle grow very fast & it won't be long before he needs a bigger tank, but it sounds like you are already thinking about that anyway.
Glad that you are here on the board to find out what you need!
Welcome!