Here's what I'd do. First I'd soak him in luke warm water for a bit, but only enough to cover the lower 1/4th of his body. That way he can soak without having to swim. Then I'd place him back in his enclosure. Now I'm going to assume that you have him in either a tank or some sort of cage. If he's in a tank, cover most of the sides so he isn't stressed by the outside world. I would put him on some sort of bedding, but that's just my preference. Make sure he has a hide on the hot spot where he can hide, but it's nice and tight. Crank temps up to about 88-89 degrees on the hot side and leave him be for a few days. Then I'd offer him a mouse, small enough for him to eat with no problem, but large enough that its movement may catch his attention. Now if you're going to assist feed him, I, too would give him a small meal that is easier to push down.Even though it'd be a small meal, it's better than nothing.
Pics of your setup would greatly help us to see if everything is on point.
PS: don't worry about the drinking water being too cold, as long as he's not sitting in it for extended periods of time, you're fine.