Maybe try to adjust his drinking schedule? Encourage the big drinks in the morning, and make the supper time drinks a little smaller. (I know the feeling though, of just being happy they are drinking - my girls don't take in as much as I think they should, either.)
And if that's not working, try giving him a clean nappy when you go to bed. Then at least you don't have to get up to do it, and he gets a few more dry hours, hopefully!