Axolotl floating and not eating, no change after full week in fridge

So a few weeks ago, the younger of my two Axolotls took a bit of a turn and stopped eating after some overfeeding caused a water-quality drop. I did daily water changes and the water quality is back down to safe levels, but he's started floating almost constantly. I know floating is often a sign of impaction/gas, but the weird thing is that if he gets spooked he'll swim down and burp, but half an hour later he'll be back to drifting around on the surface. Last week I tried fridging him and every time I checked on him and change the water he was on the bottom, but as soon as I put him back in the tank he's gone back to floating.

He was always a picky eater and refused to eat earthworms like his tankmate (spat them out no matter how small they were cut), but until this all happened he would happily eat a few pellets per day. Now he's barely eaten in a few weeks and I'm worried that the poor guy will starve.

Any suggestions as to why he might be choosing to float and why he still isn't eating?

What are your current tank water parameters

Don't fridge him make sure the water is at least 64 degrees constantly you have to get a chiller they run a pretty penny but saves you the hassle or put him and the tank in the room with air conditioner. They don't like temp changes and a few degrees will cause stress. I have 6 axolotyl 2 blue filled adults 2 juries and 2 babies and my blue gill just laid eggs so ill be welcoming a bunch of blue gills. They all eat different I feed the babies blood worms the two juvies and one adult like earth worms . the one adult does not have such a big appetite she eats blood worms and small pieces of. Cooked shrimp. So no fridge! Keep at steady temp 64 and try new things to feed him,roaches,live ghost shrimp,cooked shrimp or worms they all like different stuff. All mines hate the pellets? good luck

I can say with full certainty that temperature is not the issue. My tank's ambient/room temperature currently hangs around 14°C (less than 58°F), so a cooler is unnecessary.

I've just done a water test and my parameters are:
Ammonia 0
Nitrite ~0.25, maybe a little over
Nitrate 5.0

I'm still doing daily 20% water changes to knock those two down to 0. Are they high enough to be causing harm?

Nitrate is okay, just keep up with those water changes to bring nitrites down and stop nitrates going higher. Was your tank fully cycled before this happened?
Is his full body floating or just the back side? Sometimes my axolotls float for the fun of it, but if it's mainly the backside it could be gas.
What kind of earthworms are you using? I recently found out that red wrigglers can have a nasty taste and some axolotls don't like it. I've been struggling feeding Bucky as he keeps spitting them out, so I'm sourcing some nightcrawlers.

Yeah the tank was definitely fully cycled before this; it's been running for about 6 months now.

He's full body floating. What I don't get is why he would do it constantly. Like I said, if spooked, he might sink for a few minutes, but before I know it he's back on the surface. He gulps very often too, which would make me suspect low-oxygen if his tankmate weren't fine.

I'm using European nightcrawlers (Dendrobaena worms) to feed the other axolotl. Before he stopped eating full stop, the problem child would occasionally snap at any that fell past his face, or go for one that spent a long time wriggling at his feet, but would promptly spit them out.

I'm not sure where to go from here. What do you have in your tank for oxygen? Filter, sponge filter, air stone, etc? I don't know if the oxygen levels could affect individual axolotls differently?

Sadly the poor little guy pass away last night. I wish I could've known how to help him, but at least he isn't suffering anymore.

so sorry for your loss