Mention statistics and how you heard that many people are actually bisexual. If it's positive, then you can explore the subject even more. Casually mention that this guy is gay but that you don't have an issue with that kind of thing. Talk about your mate at work and his bizarre antics.
Be a little mysterious with a hint of sexy. However, don't make downright obvious hints that give everything away.
Always refer to your ex as your "ex-partner." What you're doing here is dropping hints. If he doesn't bring it up, make sure YOU do.