[Uh. How does he tell her that he knows pretty much the entire student body back at their school?]

...Enough women to know what I'm talking about! Don't worry about it!

[Real smooth, Ryoji. Best to focus at what's at hand here!]

Just as I thought, you look stunning! Here, let me show you -

[Ryoji may not have a mirror, but what he does have is a device with a camera! And it's very easy to show Mitsuru what she looks like with the tablet screen.]