Honestly, I'm getting danger vibes from this one. Poly is an interesting way to build our life, and can work, but this is not the way. A few solid rules of thumb when considering opening up a previously existing relationship.
- First, it should be a discussion between the two people in the relationship. The desires of your friends shouldn't come into it (except as a potential source fo threat). "Wary but willing to give it a try" sounds like your boyfriend wasn't the one to broach the subject, and the fact that you're worried too suggests that the best friend might have been the one to broach the subject, and that just sounds like a poor choice all around.
- Generally speaking, the relatively healthy way to move into poly is that the person who is pushing it lets their partner go first. As previously noted, if neither of you is pushing this thing, then you shouldn't do it. Likewise, if the person who is pushing this thing isn't willing to let their partner go first, then the parts of them that say that they want to do the viable poly thing don't really mean it.
- Long-term healthy poly relationships pretty much require that you decide who your primaries are, who your secondaries are, that the primaries are all totally on board with each other, and that you be willing to ditch any secondary for any reason or no reason based on the feelings of any primary. This guy is your best friend. Are you willing to drop him as a partner without hesitation, recourse, or bitterness just because your current boyfriend says so?
On the flip side, if you're still, say, in college... well, it is a time for experimentation - and after all of the excruciating drama has past, and you've (most likely) either switched to your best friend or washed your hands of both of them you'll have learned a fair bit about yourself and how you work and whatnot.