My situation nearly 20 years ago is similar to RIchie's now. I disassociated myself when I was 19 years old, in 1988. My father is not a Jehovah's Witness, but my mother was, and still is. While I realize that my mother was deeply hurt for a several years after my disassociation, she never shunned me.
I continued to live at home for several more years while I finished college. And when I moved out to go to graduate school and begin my career, she welcomed my visits, shared meals with me in her home, my home, and restaurants. To this day, when we get on the phone we can talk for hours. We talk about everything except religion. We talk family, we talk politics, we talk clothes, food, health, television, movies, and sometimes we even talk about legitimate family business.
You see, my mother believes that maintaining familial relationships is family business. My mother believes that sharing meals together is family business. My mother believes that my visits to her, and her visits to me are family business. Maintaining close, loving support for one's family is family business. No one outside your family has any right, or business to judge your continued relationship with your only son. And you would not only be a disgrace to motherhood, but a disgrace to the true meaning of Christian love by excluding Richie from your life, when he leaves your religion.
If you experience ramifications from the congregation for continuing your relationship with your only child, don't hesitate to invoke the explanation I am sure my mother has resorted to in desperate times: "His father is the head of the house, and he insists I continue to be a mother to our son. Richie's father knows that Richie will always be there for me, and insists that I be there for him." Trust me, Mom, you will be losing far more than a son by cutting Richie out of your life. You will drive an unnecessary wedge into your marriage. It will serve to cast a divide between father and son, and ultimately grandfather and grandchildren should you decide to cut off Richie.
Don't do it. It isn't necessary. You have an ideal situation. You can blame your unbelieving mate for your continued relationship with your son when the friends in the congregation try to manipulate you, and you can know in your heart that you are conducting the true essence of family business. Love and a common purpose of living happy, productive, intermingled lives.
And besides, I bet Richie intends to take care of you when the time comes, like I intend to take care of my mom. Who else do you have who will?
It's ok, Mom. My mom has survived 18 years of my disassociation, and will survive it until she dies of old age. You will survive too.