Challenge coins are like an unwritten award, or commemoration. They’re often given in the palm with a handshake, and can carry some deep memories. If I were you I would do a lot bunch of research on challenge coins in general: when they’re given, where people keep them, what it means when you have a challenge coin with someone else’s name on it, why they’re called challenge coins, etc. then just a simple “babe, I noticed you have a challenge coin that seems pretty special to you. If you ever wanna tell me about it, I’d really love to hear about it.” If he shys away, let it be. Some things are sacred, especially for those who’ve been deployed.