I never really had the threesome fantasy, but there was one night when my older best friend took me out on the town to do the crawl shortly after my 21st birthday, then we went back to his place and laid into a bag of, um, extra pure hemp. My girlfriend showed up to drive me home, and she laid into the goodies too instead, and we both had to crash there. We collectively got the idea that it would be cool/fun/different/whatever to have a three-way.
It wasn't.
I never had a paternity test, and I'm 90% sure my 14 year old is really mine, but I can never be totally sure without a paternity test. She conceived right around that time, and we only had one condom, not two.
I'm not pissed at Mike about this, because we all came to the same conclusion by mutual agreement, but he and I have barely spoken five words in the past 14 years, and our relationship was never the same after that.
I think the whole thing is a really bad idea.
If you're not down with that, then definitely tell him to get over it. Feel free to tell him my story as an example of why it really seems like a horrible idea the morning after.
(And I never touched the, um, hemp again after that either.)