Hello Loveme46,
Perhaps you could say something like, "Honey, I have been wondering if we could try polyamory, could we talk about that?" If he already likes multiple women, he may be ready to try poly. Just be aware that many people consider cheating a lesser evil than polyamory. Something about hiding an affair, it's like an admission that it's wrong. I, of course, am totally okay with polyamory, but I can't predict how your boyfriend will react to the idea. I do think you should ask him.
Sincerely,
Kevin T.