Hi Carrie,
My 41 year old brother was diagnosed in July with this too. It just went into his lungs within the last month. He goes into the Univerisity of Ann Arbor, MI next Monday. They are trying a treatment called Interleukin-2 (IL-2). Its a chemo-biotherapy. I've got some info on it if you want me to fax it to you. The doc did mention that if the cancer had spread to his brain that they would be taking another route, so I'm not sure if this would help your step dad or not. With this, they go in the hospital for 5-6 days of treatment, come home for two weeks, then back in. It goes on for 4 sessions, which is all the body can take I guess. Theres a 50% chance of remission.
Other than that, I'd say ask your side of the doctors if this treatment would work. Its still experimental, so who knows.
They told us that advanced melonoma is a very fast spreading cancer, which you probably already know. So, enjoy the time you have with him and make this a great Christmas as a family. We will be too. God Bless. Shelly in Michigan