Andddd... I have a floater!

So, I 'adopted' an axolotl from my sisters boyfriend. They each got one this past September. My sisters axolotl is doing great! Growing, eating like a champ.. Etc. The one I've adopted has barely grown at all. He couldn't get the poor thing to eat. It was stuck under another tank, in warm ish water, behind closed doors.
I've had it for about 2 weeks now. I've been getting it to eat nightcrawlers, and it's had 3 wax worms. They just have to be cut up tiny, and soaked first or it won't eat them. It's not eating much, but it's something.
In the two weeks I've had it, I've only seen one poop. And I noticed last night, the poor thing was having a rather buoyant bottom. But it was still moving about, not looking too stressed. Today, I got up to check on it, and it was floating on its side, on the top of the tank. I assume it's gas, or it's constipated. So I've put it in the fridge for the time being. In clean, dechlorinated water, just deep enough to cover it, with the light off, and two thermometers in there. One in its water, and the other beside the tub it's in.

My question is, is there anything else I should be doing to help the poor thing?
Any help/advice would be appreciated.

For the moment keep him in the fridge until stops floating. It is likely gas or constipation like you said. What kind of substrate is that in your tank?

I have it on the same sand that my sisters is on. It's some really fine aquarium sand.
The original owner had it on black sand, and said it kept spitting its food out. Knowing this, I feed it on a glass dish to try and prevent it ingesting too much sand. Idk what his problem is, I've been getting it to eat. We've managed one whole worm, and 2 waxies in 2 weeks. It's not alot, but a chunk of worm a day is better then what it was getting before, which was apparently nothing.

How is the floating going now?

It's bum is still floating, but it's pooped Twice now, so I think that's a good sign. I've taken it out of the fridge because where I am is cold as heck right now anyways. And I'd like to put my food back in there, lol. I just put the container and the axo tanks near the door. I'm able to keep a closer eye on it today as it's my day off, so we'll see how it goes. I'm gonna add a bit more water to the container on today's change and see if it can stay at the bottom yet.

Great! Pooping is a good sign, keep doing what you're doing. If the floating doesn't go down or gets worse pop him back in the fridge for a bit more.