Also, read this: https://www.amazon.com/Eight-Things-Known-About-Polyamory-ebook/dp/B00JY2PLI6
It will give you some good guidelines on how to make it safe for the person you are bringing into your relationship. It's short, to the point, and a great read.
Ditto that about More Than Two. But if you'd prefer an excellent short book, here's Cunning Minx's Eight Things I Wish I'd Known About Polyamory (Before I Tried It and Frakked It Up).
If he's put off by long books, a much shorter book that covers the ground well, with snappy writing, is Cunning Minx's "Eight Things I Wish I'd Known About Polyamory (Before I Tried It and Frakked It Up)": https://www.amazon.com/Eight-Things-Known-About-Polyamory-ebook/dp/B00JY2PLI6