Welcome to our forum.
You might start by telling your boyfriend you'd like to learn more about polyamory. Then you might try something like Opening Up
, and read that book together as a couple. See if he would be willing to get a membership on this site, and then you can both post questions when needed.
I guess the main point is to go slowly, and give him some chance to develop some familiarity with the subject, so that he can explore his thoughts and feelings about it more deeply. Have a look around on this site and see what threads call to you.
Hope this helps some.