John, beside the fact that you handled the situation as a true professional should, don't worry too much about him. You can always ask him up on stage to do a number by himself, so many audiences members think it's cool to play along, but would never get up on a stage themselves. Sometime they need to know why you are the one getting paid to be on the stage :) Of course this is a judgement call, because he may not get it, even if he screws up on stage...but we have all been through this where your audience thinks they can do what you do. I work in interactive street theater, and also am a musician in the forum, THAT gets interesting. Everyone you meet has played some instrument way back in high school, and wants to work with you, you can only be poliet for so long before you have to explain it's your career annd you are very serious about it. I also know it's very hard to be a a musician starting out, because so many people say they can do thing they really can't.
Good luck Jill