It's markings and colours look like that of a gray tree frog that's been sat on something brown. But my word, that's plump for a gray. lol Does he ever change colour when sat on different coloured decor etc? If so this is a trait well known for the gray tree frog, hyla versicolor. They're masters of camouflage, my babies are really good at pretending to be cork bark. lol
I wouldn't be so quick to rule out the Japanese treefrog. While the green form of hyla japonica is more common they do come in many shades and patterns including grey