Jo, Has he ever been seen or known to take a bath at all? I can always tell by the wet paper (and everything else in a 2 foot radius of Pritti's water bowl, which is the only place he'll ever bathe unless I give him a shower in bathroom).
You could see if he'll take a bath in it. If so, remove it as soon as you can from when he is finishes. Put it in a different spot than it has been in. Splash your fingers around in it, make some fun noise and see if you can get him interested in doing the same thing. You could also just remove it now and put a sufficient dish in the bottom of the cage for him to bathe from -- like a pan with a half inch water and see if he'll stand in it. Or a heavy bowl where he can stand on rim without it tipping over. You can even put one in each corner and see which he might like to use. Just be sure they aren't filled too high 'cause he's a little guy.