I like it. I usually think of it as a GP or a mn (or sometimes a nickname). I like it; but I do think it's a bit short. It's like Max; it isn't a nickname, but it's so short that it feels like one. I'd give him a long middle name he can use it he wants to later on.
I like Hazel on a girl, but I think it might be cool on a boy. I definitely like Haze.