My son has Asperger's Syndrome. I was relieved by the diagnosis, because I had been expecting it for a long time. He was always a very "different" child, and I knew something was up. A diagnosis meant finally getting school support and having answers regarding my son's more unusual behaviours.
Any parent who believes such a diagnosis is the end of the world needs a good smack. Kids do not come with guarantees. They do not exist to fulfill our dreams or give us someone to live through, vicariously, later in life. ASD is not cancer or a spontaneous brain injury that radically alters the path of a child's life. There is nothing to grieve. This is who your child is, and as their parent, you'd better damned well accept and love them for who they are, not grieve who you hoped they might be one day, because that fantasy child never existed. Even profoundly autistic adults can go on to lead happy, meaningful, lives, and contribute to the world in their own way. A happy, healthy life is all I want for my son, and a happy, healthy life starts with the love and acceptance of your parents.