Here's the problem to start..The iPhone has limited power and an inexpensive internal Dac feeding the output..The wrong $500.00 headphone will be wasted...He needs something fairly easy to drive and not overly resolute...You also need to determine if he needs the sound to be isolated from disturbing others,which means he will need a closed back headphone.A closed unit will have slightly better bass response but will also limit the sound staging..
An open back headphone will leak sound but will have better sound staging and an airier sound...
For a closed back headphone that will sound decent off the iPhone I recommend the FiiO FT1 or the Meze 99Classic...
For an open back the Sennheiser HD660S2 would be my choice...
Also introduce him to head-fi.org,where he will find an entire world of headphone information....
Lastly but what I consider to be ABSOLUTELY MANDATORY is for him TO EXPERIENCE LIVE MUSIC!!!
If you don't know what real musical instruments sound like,you will never be able to judge how music reproduction instruments like headphones are actually performing....
