In terms of how to start the conversation, I found the resources at www.morethantwo.com
to be immensely helpful in sharing the concept that loving more than one person doesn't make any love less. I agree with the other posters who have said that you owe him honesty, which seems to be what you yourself also feel. Give him the chance to understand that this doesn't change anything, that you still love him and are committed to him exactly as much, but that you don't want to lie to him or have to pretend that your extramarital liaisons mean nothing to you.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.