My best advice is just to talk to him about it, without expectations. If he is not comfortable with it right at the moment, at least you brought it up as something you would be interested in pursuing. A lot of guys, myself included are often shocked or taken aback by something my wife may say or suggest we do, or something along those lines. But once I have time to ponder or think on it, the idea or suggestion she put out there might actually be interesting to do. Your boyfriend may initially be a little shocked, especially if poly or opening your relationship had never been discussed before, but maybe its something he' thought about in the past but never brought it up...
I also have to say, personally, I think venturing into a poly relationship is much easier to do with someone who is already friends with the both of us rather then trying a new relationship with a stranger.
Good Luck and let us know how things go.
The characters in another chapter we call life!
T - Wife - Poly
J - Husband Poly