I would just tell him that your Aunt said she's keeping everything a surprise, so you're in the dark. If he brings up buying you something, maybe just brush it off and say oh that's so sweet, you don't need to do anything. If there's a good way to work in where you're registered, you could drop that info, but it would seem kind of weird to say I don't have an invitation for you, but you can go buy us some stuff at BRU.
Hope he just drops it, though! Doesn't sound like the kind of guy you want to spend an afternoon with!