Some thoughts:

-Slytherin faces a stigma post-war. Everyone just naturally assumes you're an asshole. Gryffindor kids, especially, like to play mean pranks and bully the Slytherin kids - particularly those without any Muggle parents. This would be a point of contention in the campaign, and help show that not everyone with a pretty face is a good person. Play on the established tropes of the universe.

-The campaign itself will start out very simple. Like, very simple. This is meant to ingratiate him into Roleplaying as a hobby, and allow his (obvious) creative leanings to shine. For that reason, we're going to have some minor things to which his character will attend: Bullying, pranks between houses, etc. The only great evil to defeat would be those rude kids next door.

-Lots of familiar faces. Hagrid, McGonnogoroth, Evil Janitor and all the ghosts.

We'll be using Fate accelerated for all this, as I think it'll help him jump in without getting overwhelmed by rules. Wahoo!