You don’t get to be a “Patron Saint” of something till you actually make it to sainthood. Blessed Father Casey has just been beatified; he is not a saint yet, as that requires the next step, canonization.
So he’s not officially a Patron of anything yet, and won’t be until he becomes a full-fledged saint, God willing.
He especially loved poor people and sick people, so those were two things important to him.
I think the city of Detroit already has adopted him as their informal patron saint as he is certainly bringing a lot of interest and people to a falling-down city in need of visitors.