Welcome to the forum...
Sounds like you are off to a great start with your little guy. It would be great if they'd stay snall forever, but knowing that won't happen is a given.
Just be prepared for the size of his home to grow as he does.
You probalby will find out like most of us on here that the gravel in the bottom isn't the best solution. #1) Turtles have been known to eat it causing major health problems, and #2) even if it's bigger than his head but covers the entire bottom, it's a huge pain come cleaning day!!! (I'm speaking from experience here
) The best thing I ever did was to ditch the rocks that covered the bottom of my tank and go to a few river rocks. MUCH easier to clean!
Like, turtlesirens said, fill the water up. (RES are aquatic and love to swim; the more room the better) And invest in a floating dock or look in the photo gallery for ideas on how to build one. This takes up less swimming space and is easy to deal with during cleanings.
If you have any questions just ask....