A Son who wants no contact

Hello out there,I have just found this support group. I feel sad today my life is very hard and has been for a long time. The problem with my son has been happening for many years. He wants no contact with any of his family,Ibelieve that he has a mental disorder that he refuses to accept treatment for. It is I think a social phobia of sorts.He has had no contact with his sister while she was having treatment for breast cancer, and when her marriage broke up and she was left with 3 young children.His father has aml now and he shows no interest is reuniting with us. I am in need of some advice as to what I can do to help this situation