thank you so much for your comment. I do have water bowl that he can sock and deep substrate(may not enough though^^; and i give him at least 2 times a week with natural sun light ( without window block ) in wide water pool he can totally sock up.An enclosure by a window is not the best idea. They can over heat quickly as you can't control the heat. They also don't get any UVB from the sun as it won't go thru glass and screen.
Your Boxie needs a water source to get into. They like to swim. At least a large bowl he can easily get in and out of. Also the bowl you have in there needs to be lower sided. They also like to bury themselves. A deeper substrate would be great for him.
Beautiful turtle.