I'm sorry I don't have any advice, but I just wanted to wish you luck and say that the little guy you picked up is precious. <3
I would just be careful about your substrate - I would probably replace it with fine sand (or nothing), personally. If they suck enough of those in while eating, their insides can get impacted - and if you can't get the waste out of your system, it eventually poisons you and you die.
My axolotl looked similar to your when I got her. I was very nervous for a long time, but she is doing great now ! I just fed her lots of good food to fatten her up. Yours looks skinny like mine did. I fed her sinking carnivore pellets and nightcrawlers and sometimes blackworms. For a little axolotl, you will have to cut the earthworms up, mine still cannot really manage a whole one ! Also, be careful not to feed too much, emaciated animals can be "killed by kindness" - just start of easy until you see some fat/body mass start to pack on.
Mine went from
(skinny, eaten gills, one arm)
to
It took a few months but looking back it's so amazing the progress she made !
Just watch out for fungus - like, it looks like a little white cotton ball type of fluff. If you see that on any exposed wound (or elsewhere), you might want to check out the salt bath tutorial.
Whoops, guess I had some advice after all.

Good luck !