The coffee idea is good, maybe no need to "plan" it but just take the opportunity when you see him. This calls for a bit deeper discussion than you can have in a quick hallway discussion.
I tried it at the end of a fun day with a friend, as we were parting ways. Complete botch, and in that context no way to recover. "But I thought you were married!" "Uh, yes I am, and that's not going to change..." End of conversation. Obviously I don't have a lot of practice at this. Naturally she thought I was asking to cheat with her
Luckily she was still willing to talk to me, a week later I got to explain the situation properly. She responded with curiosity and good questions, and by the end was willing to give it a try.
(That "try" never actually happened, but that's another story)