Some frogs are just silly! If everything is in good range, and as you said, he's eating and active, no worries! One of my greys sleeps in the water almost all the time, no idea why, he's just a weirdo. I was really concerned at first, especially when he got bloated from the water (I'm sure there is a thread on here regarding that) but it's been 4 years and he's perfectly healthy.