Whatever it turns out to be, it's a CUTIE! I have enjoyed my "turtle education!"
As my boxies have grown from hatchlings, they've gradually taken on different skin colorations consistent with sexual dimorphism [see my avatar pic -- that's a male]. And of course the coloration of the shells (top shell, carapace and bottom shell, plastron) has changed with each baby which seems to be a reflection of a mixing of 2 or more EBT groups.
As hatchlings, my babies pretty much all looked alike, and were perfect little camoflauge machines. Their shells bore no resemblance to what an adult EBT turtle would look like. I don't know about water turtles, but would imagine it would be the same. Would love to see what your little guy's shell looks like in a couple of years [if you can get him to stick around]. He certainly has a distinctive and pretty skin patterning.