Probably a jersey would be the most standard thing.
You should first check if he already doesn't have them, but a Dr. J (Julius Erving) jersey or a Moses Malone jersey are vintage stuff.
Or if you can get your hands on them, a ticket for any game of the playoff series developing between the Sixers and Detroit would be gold.
The schedule is as follows, though I'm not sure how you get them and someone else might help you with that. And Game 5 will only happen if Sixers win one game (any).
Game 3 - Fri April 25 Detroit at Philadelphia 7:00PM ET ESPN2
Game 4 - Sun April 27 Detroit at Philadelphia 7:00PM ET TNT
Game 5 * Tue April 29 Philadelphia at Detroit TBD TBD