I am sorry you are going through this. I don’t want to hurt your feelings more by saying this but I feel like talking to him about an open relationship just to get him to open up and then telling him that you can’t be with him in a romantic way after that did you nor him any benefit. I have to admit I am also a jealous person when it comes to my husband and in the past I’ve done the same when we had similar issues. I would get him to open up and then blow up in his face for opening up. However, from your husbands behavior it does seem like he has no intention of wanting to leave you or pursue anything with his friend. It’s hard but try and focus on your health as you have a lot to look forward to. Hoping and praying things work out in your best interest and may you heal from this.