How are you giving him showers? Are you moving him into a bathroom to do so or are they in his cage? Also, are you using cold water?
In my limited experience with Meller's I have found that they don't like to be moved, pretty much at all. Not always but most of the time, and I feel that my female loses all track of what she was interested in doing if I move her because she gets nervous and distracted. So if you can manage to keep him in one place permanently as much as you can, do it. Whether it means moving her permanently into the bathroom or what, but she won't drink and drink well if she doesn't feel stable and safe. The more you move him around the more he might be stressing out and the more the parasites might be taking over.
Also, all three that I've had seemed to be very temperature picky, as well. Very Goldilocks, really. The water couldn't be fresh-out-of-the-tap cold but it couldn't be warm either, they all liked it a little warmer than luke-warm. It seems like a silly detail but Meller's seem to be very persnickety so you have to indulge them.