Vets-asap! It could be he has an underactive thyroid which will make him drink a lot, one of my old cats had this and it worried me the size she got to! The lethargy would be because of this aswell, it's important to just make sure he has enough water around the place-don't cut down the food though, keep everything normal and seek advice from a vet immediately.

It could also me diabetes, that would explain the drinking, I haven't heard of them ballooning with it though, definately the vets for the kitty though.

Good luck and hope he gets better, keep us posted! If it is thyroid-related and you want to chat to someone who also had a kitty that suffered you can PM me or reply here and I'd be happy to chat about my experience with it.

__________________
*Katania aka Shin*

Mother to the best cats:
Sooty (5yrs)
Smokey (4.5yrs)
Bandit (3yrs)