Hey, I'm a mathematician not a psychic! (Yes, I can prove that. I think, therefore I exist. Psychics don't exist. Therefore I'm not a psychic.) But here are some hints.
 He would probably have remembered if he had repeated one of those digits.
If he agrees that he did not have a repeated digit,
that rules out some possibilities (there are 7x24 = 168 left.) If he thinks he did, that narrows it down even more to 3x6 = 36. If there isn't a lockout feature, he can try all 204 possibilities in under an hour. He might want to write out a list first.
 He may be able to remember something about how he picked the number. I don't have an Ipod myself but I know it uses a "ring" directional controller. Thus, entering numbers is presumably done by moving a cursor over a grid of numbers and clicking on the right ones. "Easy" patterns are ones with a simple geometric shape on the grid.
For instance, on a touchtone keypad (the most likely layout) 258 run down the middle. Obvious extensions of these to 4 digits would be:
2580, 2587, 2589, 1258, 3258,
and the same ones backwards. If that doesn't work keep trying the same idea.
Good Hunting!
RD
