Okay cool we can go with the dad thing/surname thing. That'll make for an awesome awkward start to things. But Zack will think he's a cool dude anyway.

Zack has a few memories of Seph, but no names yet. So he might think Seph looks familiar but at least he's got long black hair instead of silver to throw Zack off a bit.

(ib4 Zack drags Seph out for drinks all the time, please be his friend?!?!)