I'm very sorry to hear that. He looks pretty young to me; I wonder if maybe he was just too young and died of natural causes. Please bear in mind that I am no expert at all! I just am speculating as I have gotten to watch (and occasionally help) an axolotl breeder and their assistant care for a single clutch in the past, as well as raised a couple young frogs myself (with their help); but, iirc, there's certain die off rates that happen even with impeccable conditions and care. That's part of why breeders tend to wait to sell them after various ages, although I don't know what that age range tends to be. It seems that, with amphibians, not all of those that hatch make it, even without predators and in perfect consitions, which is probably part of why they have so many babies in a clutch.
I'm sorry for your loss. Again, there may be something that I am missing or am incorrect on, but I'd think that this may have just been one of those things that happens, and there wasn't anything you could do.