SS, I have belonged to another support forum for 9 years. It's for stepmothers. Lovely isn't it? How my need for support groups grows. Anyway, I have known of at least 3 situations in which the children have slipped away from their parent(s) and stayed gone for sometimes a few years. Each time the children have returned with love and newfound appreciation for their parent(s). This last time, even I wondered if this woman's son would return. He let her know he hated her with a venom before he cut her out of his life. After a few years, they live together and have a much better relationship.
So, while I'm brand new to Al-Anon, I hope I can offer you hope that your son will come around. I hope that you'll enjoy many years of joy with him and that he'll realize how much he values you. I know you worry now. I have no advice on that. The women I know who went through it worried too. Lots of hugs and prayers were given. These folks seems to have some good specific advice. I'd listen to them. Ultimately, I know you are hurting deeply and hope you are able to carry on in peace until he returns.