76 is WAY too warm for him. Your tank should be 60-64 degrees and should NEVER go over 70 degrees. That's probably not causing the bump, but it's definitely not helping any- warm temps really stress our axolotls and make a LOT of things worse for them. It'll be a lot harder for him to heal. Also, the temperature may be the reason he's not eating.
A lot of people refrigerate caudates to fix all kinds of problems, and it's a really commonly suggested solution for axolotl health problems. Put him in a LARGE plastic container, like a 1 gallon ice cream tub. Fill it about half way with water (at LEAST 1/2 gallon of water for an adult axolotl) from his tank, and put the whole thing in the fridge. The cold temps will slow down his metabolism, slow down the growth of any bacteria, viruses, fungus, etc. . . on them, but doesn't slow down their immune system. Change the water once a day, and don't worry about aeration or filtration for it. When you change the water, use pre-chilled, dechlorinated water. To do this- dechlorinate a gallon of water & keep it in the fridge with the axolotl. You can feed the axolotl after 3-4 days of this, but it may not want to eat. I've always heard of keeping the axolotl like this for at least 3-4 days, lots of time a week, but I've heard of people keeping axolotls in the fridge for weeks at a time. I've never had to refrigerate any of my axies, though, so I can't speak first hand. This is just a solution I've heard of, and that I'm prepared to try for my axolotls if they have health problems.