You did the right thing. It's not your place to correct him on any uniform issues; that's the job of his superiors. Passing the word through the military authorities is the way to go.

Even though he is not covered by military regs, I'm sure that he has standards he must meet to progress in the program (CAP was mentioned and that's the way it works there). If he gets in trouble about his uniform appearance, it could negatively affect a future promotion, and from his attitude I say he deserves that.