Glad it doesn't seem to be a medical problem.

Have you considered "restricting" his water before bedtime and
when you know he'll be alone and not able to take a "pee break"?

I'm very lucky - the FurKids KNOW when I'm not with them that
they shouldn't drink alot...
VERY little water is consumed when I'm gone!
Upon my return, there's usually a "rush to the bowl" and
they "tank up" knowing The Doorman is around to let them out.

/s/ Phred
..