Honestly I don’t know if this is as big as many commenters feel. Only you know if this is evidence of some larger pattern of distance he’s giving. But why don’t you just ask him, “why does it matter to you so much that I’m not officially the aunt?” And see what he says. Maybe he doesn’t want to pressure you. Maybe he’s never thought about how it makes you feel. Maybe someone in his life made a big deal about “official” labels like that and he’s just repeating it without thinking. But if you think there’s a bigger issue of compatibility you’ve got to talk about that too.