Crate training helps a lot. Dogs generally don't want want to pee in their den. Our dogs are trained to "go to bed". We do let our dogs sleep with us on weekends sometimes, but it's a treat not the norm. If he is drinking a lot of water before bed that may be it. It may be that he isn't getting it all out. It could also be more serious, maybe a UTI or something. I'm not a vet though. I'm just dealing with a 14 year dog that is doing the same thing.
His eating and drinking habits appear to be fine. What I suspect might be happening is he has just kinda figured out how to lift his leg and maybe he is holding it in because instinct is telling him to mark territory and he hasn't figured it out yet. Dog's are sometimes known to hold in urine simply for the purpose of marking territory. Well just a educated theory after much googling. Thanks for the comment :-)