(I have a fantasy about wearing his black t-shirt around his room with just a cotton bra and panties under it.) ;)
Most Helpful Guy
In his place I'd be open to this - I like this aesthetically and it suggests devotion.
If you're anxious, you could trade items of yours for items of his - though try to make it something that he can use or something that can be a comfort like an object that either smells like you or a scent that you use frequently.
Most Helpful Girl
Just ask him nicely, I mean don't ask for everything at once but i'm sure if you just ask like " hey babe, i really like your shirt could i borrow it sometime?" he'll say okay.
and as for your little fantasy, I do that with my bf's favorite hockey sweatshirt, so comfy and so fun to sleep in