This situation is rare for me, too, but it happens. I have a few routines specifically designed to use guys from the audience -- a cut & restored rope routine is my favorite by far, but routines using wallets & money are natural fits for interacting with men. Gambling themes are also good, particularly if you can ensure a win on the volunteer's part.
1) See if you can spin the gentleman's appearance onstage with you as more of being a "participant" than an "assistant" -- don't use the word "assistant" but rather a word like guard, inspector, partner, teammate, locksmith, warden, sidekick (possibly), etc. A descriptive, participatory role seems more manly to my ear. Then again, building a routine so the audience member is ACTIVE is probably the better approach for any audience, but especially when working with guys, I try to let them DO something rather than just stand and wait for me to do something.
2) See if you can involve two or more men at the same time, rather than just one -- more of a "team" assist rather than an individual appearance. It's even better if they get a chance to shine. Maybe they get to tie you up, verify that the locks are real, tie their boss to a chair, etc. -- I try to look for cooperative tasks or tasks that can be done by multiple people.
Those are just off the top of my head. In my shows I use more ladies onstage than men, but I tend to borrow items more frequently from men than from ladies. It does takes some thought to reframe a presentation for an all-male audience. Good topic for discussion.