Sometimes they burrow down to the bottom of the cage to pee and you don’t find it for a while.
It sounds like a lot of water. I suspect if he seems fine in himself that he is peeing and the bottle may be leaking. It could dry out overnight where it’s leaked. Have you actually seen him drinking a lot? It will be a process of elimination - try another bottle as well as this one (ie two bottles in the cage) - bottles are notorious for emptying themselves or jamming up. I use the mouse sized ferplast drinky bottles - best ones I’ve used.
Also maybe add another litter tray with chinchilla sand in and see if he uses it. If he’s hoarding in his pee corner he may have decided to pee somewhere else. Incidentally they tend to pee on their hoard to deter people stealing it. Then you have to remove it because it’s pee soaked which will make him pee on it more
It can become an itch scratch cycle. Suggest removing the per soaked hoard (leave anything that is dry) and replacing it with the same amount of new hamster food next to his pee area. Eventually that gets the message through sometimes! Also don’t clean out too often - just spot clean.
If you see any signs of swelling or him not being himself though, take him to the vet.