I think I read the haircut quite differently, maybe because I didn't know the ep title in advance. But I was thinking of two things -- John signifying himself as accepting the role of soldier by cutting his hair short as soldiers do. And/or mourning -- many cultures have a tradition of cutting off the hair as a sign of mourning, particularly males in cultures where hair signifies virility and power. Who he's mourning for...well, I guess in his emo-teenaged way, the John-that-never-really-was, the one that wasn't going to have to grow up to be a soldier in an unwinnable war.

What I found immensely amusing is that while I agree about the haircut signifying a Giant Step into manhood, it made him look about 12 years old :)