I'm so sorry to hear about your son, ty. (((HUGS)))
I don't have a clue how to start a support group, but maybe you could check with groups in other areas and ask how they started? I really don't know, but it's a good thing that you want to reach out to others.
We have a wonderful Friends and Family forum here that might be able to help you. Here's a link...
Friends and Family of Substance Abusers - SoberRecovery : Alcoholism Drug Addiction Help and Information
Welcome to SR, ty. You'll find a lot of support here.