Excessive drinking can be simple habit forming, which can then become compulsive.
I say this because I have a compulsive water drinking habit and all four of my children drink excessively compared to peers (albeit not compulsively). I assume from living in a home with me, who's always got a drink of water actively on the go, and gaining the habit through association.
My children (aged 5-15) drink around about 2-3 litres a day. Could be up yo 4l, rarely less than 2l. I try not to count mine, because I get triggered that I'll become thirsty. But it's more than them.
Just wanted to point out that this could be psychological and not a physical illness.
I get clammy and feel (genuinly) very very thirsty if I've not drank for 1-2 hours. I know rationally that it's not a biological need for my body, but my brain tells me to drink.
My situation was triggered by an event, which then left an imprint. For me, I was pregnant in a hot car alone and stuck for 4h in a stationary traffic jam and became thirsty, but with no access to water. This lead to me carrying a 2l bottle in the car at all times for emergency. To then carrying a 1l bottle with me at all times, to then where I am now.
I know my issue is compulsive. My children tho, they can take it or leave it. But whereas other children might not think to drink in the day, if mine see their water bottle they will always drain it (and I'll refill it sometime later).
My suggestion would be to not refill his water bottle too much in the late afternoon and evening. But then offer extra in the mornings if needed.
Also don't let him have a glass of water by the bed. I sometimes do get a drink in the night but the fact I have to get up and go downstairs for it mean a I often can't be bothered. If the water was right there on my bedside table I'd drink more in the night, then be up more to wee (and get another drink while up for a wee).