I'm not suggesting you do this, but thinking about it turns me on, so I'll share. If you run into him regularly in the locker room, observe the brand and size of his underwear. Then buy a pair just like his, wear it all day, and when you hit the locker room, do a quickie switch if you get a chance. You'll have something to sniff that night, and you'll be thinking about how he ended up wearing your underwear, as well.

I know, I'm bad, but I have a very bizarre imagination.
*Ducks and hides*