I am a Human Resources professional so I hire people for a living. As a general rule, I don't think people are looked down upon because of having long hair. I will say that people with gaudy tatoos and facial piercings will find themselves disadvantaged because many of the people selecting jobs (ie, slightly older people), don't tend to view this as favorable. He needs to look clean and well dressed. Khakis and a button up shirt should be fine for the jobs he's looking for.
Please do not have him stop by after the interview. I hate to say it, but it can have the oposite affect. I am busy all day and to be frank, this is an interruption. I often get 50 applications for one entry level job. Ask for a business card and send a note, either written or email. if you mail it, send it immediately, so it arrives 1-2 days after the interview. All you need to say is thank you and express again how interested you are in the position.
Unfortunately many teens don't present well because of a perceived lack of social skills. Tell him to put the phone away...don't text in the lobby. Make eye contact. Smile, don't slouch. Don't mumble.
Have him think of a few examples for the interview. Even if he is not asked directly for them, if he can TELL how he has addressed something in the past, then it makes the interview more memorable. Experience does not have to be paid...think of leadership in extra curricular activities, at church, etc. Here are some good things to prep for.
* Time he took the lead on something (boyscouts, school project, etc.)
* A Time he made a bad situation better
Facebook is touchy. Some people look and some don't. Most decent employers do not write people off for expressing individuality, but when someone posts "so wasted last night", it can be hard to ignore.
Good luck!!!