This sort of situation is always difficult and heartwrenching. I'm sorry to hear that you're going through it right now.
I think the key question I'd be asking your boyfriend right now is "Are you willing to take ownership and work through your jealousy?" If the answer is yes, it's possible to work through these things and have things be successful. It'll take work, but it's possible. If the answer is no, it's probably best to say "I'm sorry, this isn't going to work" and move on.
How much would you say your boyfriend knows about polyamory as a subject in general? Has he done any reading on the subject or is what he knows based solely on what you've told him? I think a good step may be to try to encourage him to do some research on his own about polyamory and say that you're available if he has questions.