That's kind of a tough situation. Telling him that you want them to change something about himself that he likes may end up hurting his feelings anyway. There's not really a way to sugar-coat it.
If you do still want to bring it up, I'd stress the "it looks bad on you" part and leave it at that. Hearing non-critical input may cause him to reconsider how his haircut looks. Outright telling him to get a haircut could make him get defensive and make him think you're controlling.
Either way, whether or not he gets a hair cut is really up to him. I don't know how attached he is to the look, but some people are impervious to being fashion policed.