Desperately wants to help
I’d sit down with Tom and tell him how much I love him, how I appreciate the wonderful way he treats my daughter, and say what a joy it is that he is a part of my family. Then I’d tell Tom his father’s behavior is uncalled for, and how painful it is to watch because he doesn’t deserve it.
I would explain that some people in this world try to control others by withholding affection and approval, and regrettably, it’s a technique abusive parents — and sometimes lovers — use to exert control over those who love them and want only to be loved and accepted. And then I would ask him if he wanted me to call his father on it, because watching it happen is painful and prevents you from enjoying the dinner.
P.S. Counseling might help Tom recognize what’s going on and give him the tools to handle his father, if he’s open to it.
It would be helpful for you to find another adult to talk to about your feelings. Because you have no family other than your stepmother, perhaps the mother of one of your friends would listen and guide you. If that’s not possible, and you belong to a church, you should talk to the minister.
Hating your stepmom isn’t the answer. She may be acting the way she is because she’s trying to cope with the loss of your father by attempting to distract herself from the pain. It won’t work, by the way, but she may have to learn that by trial and error. A grief-support group could be helpful for both of you.