I know he's just cross-dressing for Nanami's sake, and not because he wants to, but I'm surprised he's not ended up wearing some of her clothes yet. They seem to be close to the same size, and that would be a hell of a lot cheaper than him or her either one buying him girl's clothes just to keep up the act. Plus it would give more variety to the outfits he has on when he visits her house.
Maybe this is the very rare cross-dressing manga that recognizes doing that is a bit intimate when it's between a girl & guy. Or Nanami's way too shy to make the offer.