You're in the right place! Questioning is a perfectly great reason to be here.
You might start with a very gentle, generic conversation with your boyfriend, to find out what he knows and/or thinks about openness or polyamory in general. Some folks have never heard of it, and some folks have tons of friends who are poly. I didn't realize, but CBF had many people he worked with who are poly. I knew one couple, but I didn't realize there were so many in his community. Also, apparently lots of gamers are poly (who knew?), and so he knows from there too. He joked with me so often about me and my ex, FBF (who was his best friend), that I was pretty comfortable when I asked, 'so would you be okay with it for real?'
But I recommend beginning with a very generic and gentle opener. Like, 'I read this article on the 'net today, about polyamory. What do you think about it?' Or even send him a link, and say 'I saw this today, what do you think of it?'
And then all that lovely stuff what Kevin said.