I'd thank him for coming to me with his issues, and probe a bit to get him to elaborate on what "Might be a dealbreaker" means to him and where he's wanting to take it.
Because he says it MIGHT be a dealbreaker. What does that mean?
Is it a soft limit and not a hard limit? Like he's willing to try and see if he can fly with this but needs help getting the kite off the ground in a more palatable way?
Is he conflicted and needs support and nurture from you in general and then practical help elsewhere? Like do you need a poly friendly therapist to help you? To create your rights and responsibilities
framework? (That is mine) And help him as individual person to cope with his feelings at this time too?
Or is he saying he's done? Game over?
I'd think about ME and where I would want to take it and if I'm in it still or what. It's not so much about how long he's going to take. He himself may not know. It's how long I would be willing to wait for him and if my love for him is enough to sustain me through the wait. Would I still be in it if the questioning period was a year? Two years? Indefinite?
Marriage is a commitment. Given that you were clear from the onset that you were poly, and now you have a BF, I wouldn't throw in the towel just because things got rough. Maybe this BF is finally making it be a reality for the hubby -- so there's some kind of elephant in the room to break down but he doesn't have the skills strong enough to DO it with you so he's scared?
This elephant is of another color,
but I'd answer the same. BREATHE. Break it down. One bit at a time. With a pro if need be. It doesn't have to be the end.
But first answer -- how IN this marriage are you both? Didn't you promise thick and thin? Do you still love each other? Have you fallen out of love?
If so, don't put off the inevitable.
If you are still in love, talk about your Questioning Time. Maybe it's agree to a "Year of Investigation" with a therapist and then reassess after that where you are at and if enough progress is moving along to renew for another year or what.
Is your marriage worth that? Only you guys can answer these kinds of questions for yourselves.