Maybe you can put a little barricade around the water dish that only he'll fit in, and will block anything from falling in?

Say like a plastic Tupperware bowl and cut little entries at the top wide and tall enough to he can fit through them, but leave the rest of the bowl in tact. Flip it over, and it'll act as a little tent for the water bowl...

Or, if your water bowl is a part of keeping your humidity, you can get a cheap plastic strainer from the .99 cent store, and do the same thing. That way the water evaporating won't get caught in the tupperware.

Maybe there's an easier way, hopefully there is, lol... But that's what I thought of, hope it helps!