I just posted on your other thread- Yes, he definitely looks like a boy, however he is REALLY skinny, I'd be guessing as a combination of diet and probable impaction- I'm surprised no-one's mentioned those rocks/gravel yet- it's NOT a safe substrate, and chances are he's swallowed at least one which would account for the low body mass...
More questions:
Have you been quarantining the feeder fish? Although they're okay as a treat, feeders are notorious for carrying pathogens that can kill an axolotl, and additionally are really pathetic in key nutrients- Once he's out of the fridge, I'd be adding in some earthworms (Should be the thing you feed him most), bloodworms, blackworms (Both of which you can feed by squirting a turkey baster in front of his nose full of them), and pellets to his diet to get his weight back up.
As to the tank size, do you have the space to go bigger? if so, do so- the more room the better- whist he's probably okay in there, he'd most likely greatly appreciate a bit more space and definitely a few cups/pots/plants to hide in and around.
As I said in the other thread, not your fault you didn't know this stuff- the pathetic state of our pet shops and the ignorance and stupidity of their staff is to blame. We're here to help you and Mr. B through their stuff-up, so keep your head up!