First off, answer yourself, honestly - do you want this? If the answer is no, then tell him you do not. Not many people are very open in their sexuality and not many people overstep the boundaries of the intimate monogamy our society has accepted as the norm.
Yes, you might like it. Yes, you might NOT like it. But if it makes you insecure, fearful, etc. Then you know your answer. If you think you WOULD like this, but not right now, then tell him so. Just be open. :)
The questions you will ask are irrelevant if you haven't answered the first question here.