I guess it is good you found a therapist that you are comfortable with but I have read your previous posts and it appears you came to poly by accident and it was not something that your husband was totally on board with from the beginning. More like he went along to make you happy. I am not sure if your therapy is going to fix that.
The simple fact is that most people, male and female, are not going to be able to adjust to their partner developing not only outside sexual partners but also deep emotional bonds with others unless the idea is totally mutual.Your husband has had eight months to deal with this and it appears he is not one of these people, and I do not think the therapy is going to change that. It seems you are pulling out all stops to try to convince him how great poly is because you are so happy with your new boyfriend.
He may be able to handle the outside sex part, but my guess is what is bothering him the most is sharing you completely for weekends, holidays, etc.
I am sure i may get slammed on this board for questioning that everyone can be comfortable with "compersion", but so be it. Between three kids and all of life's pressures he may want a full time wife. I went through this when my wife decided she wanted outside sexual interactions. I had no problem handing the sex part, and when we started in the "swinging" lifestyle there were no problems at all. She got to act on all her sexual fantasies as long as we stuck with out pre-negotiated boundaries and rules. Then she decide she needed the excitement of the "chase" and behaving as a single person and I found myself with a part time wife. I finally had to put my foot down and tell her it was back to swinging or monogamy or divorce.
I think you would be crazy to dissolve your marraige over this boyfriend, especially if your husband would agree to let you see others if you want to without all the committment to them, but only you can make the decision. It seems he has already made his.
Good luck in therapy.