You are where you are....no point worrying about what might have caused the disinvite. If I were you I would settle down to write a really important letter. Send your husband off to his sons wedding with your love, have a lovely day of your choice, then write to your stepson....think of anything that could have made him feel as he does. When you got together you have no idea what his mum told him about the marriage or you. Speak from the heart about how you feel for that little boy who couldn’t feel at home with this new family. Tell him how you feel, how, if you love him, you maybe didn’t convey it well enough. You are the ‘adult’ here...at least for a part of this story, he was the child. Wish him well in the future with his lovely new wife...be generous in your good wishes and state that if he ever feels able to speak to you about this in the future, you would love to build bridges. He sounds to me like a boy who had his heart broken. Send this, together with a special card for their new marriage, and post so they get it on their return from honeymoon. For my point of view...I have 3 adult kids in their late thirties with their own children. I adore my husband of 40 plus years.....but my children, they will always come first. As someone once said, your children are your blood, they are part of you, your partner was a stranger you married.