My (42F) son (15F) told me last night that he had seen my husband (52M) kissing another woman

I was always close with my children, but especially my son. He always felt comfortable talking with me about everything, and last night, we were sitting in his room. He broke out into tears and told me everything. While out with his friends after school, he saw his dad (my husband) kissing another woman. At first it shocked me and it took me a while to collect myself. I comforted him and told him that I believe him and thanked him for trusting me.

We have been attending marriage counseling for the last couple of months. My husband has an extensive history dealing with depression. He tends to withdraw from me when his condition worsens. He has a terrible childhood PTSD. But he’s an amazing father. He’s always been there for our children. Also, another reason we need therapy is because he deals with low self-esteem issues. Considering that I am doing so much better career wise than him, and that I earn more money than he does, that has made him feel inferior to me.

Now here comes the dilemma. Cheating is a deal breaker for me. A lot of things add up. He started coming back home late, but he had also recently started a new job, so I tried not to think much of it. My husband had been married before me. His first marriage ended due to his first wife’s cheating. My son trusted me enough to share this with me. I don’t want to betray him. And he has never lied to me in the past. How do I confront my husband about what I know?

TL:DR: Yesterday, my son revealed that he had seen his father kissing another woman. Our marriage is already on a strain. We have been attending therapy for months. And his cheating is the last straw. We never talked about opening our marriage because we both had been cheated on before. I don’t know how to confront my husband and what should be my next move.