I remember you posted a while ago - Were you keeping all 3 in the 2ft tank, or had you moved them to something bigger? As Azhael said, metamorphosis can be brought on by bad conditions - if I were you I would be keeping a close eye on your others and the state of your main tank to make sure that the same thing doesn't happen to them. Chances are if the water quality is bad it won't induce metamorphosis, they will simply die. I'm under the impression that spontaneous morphing is insanely rare, so it was most likely brought on by something. Having said that, there is still a very very slight possibility that he is one out of a million that decided to morph naturally.
Do you:
Pick up waste and uneaten food every day?
Keep an eye on the water parameters and temperature?
Do regular water changes? - If they're in a small tank you should be doing them pretty frequently. If they're still young then it's not that bad keeping 2 in a 2ft tank, but you have to be diligent with cleaning and water changes. 3 axies in such a small space is definitely overkill - Please disregard this if you've upgraded to a bigger tank though :happy:
I'm sure you know this, but aside from the threat of impaction that Ray mentioned, having stones in the tank make it a lot harder to keep the water clean because food and waste gets stuck in between them.
Good luck with your boy, you will be lucky if he manages to successfully morph - it is very stressful and I don't think there is a very high survival rate for axies that go through this.