Normally I write big long responses to stuff like this.. In your case I’m going to make it short(ish).
Don’t do it.
Now while it may seem that I am against 3somes, I’m really not. But there needs to be a certain level of commitment, communication and time in a relationship before getting into things such as this, and unless you have been dating and living together for at least 3 years, it’s doubtful that this is a good idea.
Having said that… Your BF is right to want you to join him in things like this, and while you should be open minded about the subject, It’s probably too soon in your relationship to go down this path.
So, I would tell him that you’re open to the idea, but that you feel that you need more time together before undertaking this. Please realize however that it’s important that you do let him know that you want him to share his fantasies with you even if they are something that you have no interest in. You are at least lucky in the fact that he was able to share this with you, as many men can’t bring themselves to have this very conversation.
Good Luck..