If you are to ever be involved with both of them, it will begin with you discussing it with them. I'd suggest discussing it with your boyfriend first, as you're already involved with him.
Prior to doing so, I'll recommend having responses ready for questions he's likely to ask/concerns he's likely to have, including such worst-case utterances as "you don't love me any more!" He's most likely going to need a good deal of explanation of your thoughts and feelings and beliefs, plus work on your part to help reinforce his feelings of security in the relationship. He'll also likely need a good deal of time to work through his thoughts and figure out whether he can exist in such a relationship or not.
Then it's time to approach the friend with the idea.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.