There doesn't have to be a specific reason in canon to support why he would move to Paris! That's something you could reasonably make up in the confines of the AU itself. For instance, he could be going to school on scholarship, or perhaps a relative of his paid for tuition for him to study in Paris very early in his life. With AUs it's a blend of mixing your creativity with what might make sense for the character themselves.

Those ideas do, in fact, make sense! They're a good start to explaining some of his behavior. It never hurts to not only canon-review and watch scenes with your character and notate their personality but also look at the world around them as a whole and how it affects their worldview and their ideals.

What may also be helpful is to go through some of the other workshopped comments on this post for canons/characters you know (you can check out the directory here) and see how the players are incorporating these differences in their AUs! Additionally, we do have example applications on our applications page that display general ideas of how to incorporate an AU and what we look for in our apps!