Has your son or daughter said he or she is lonely? If not, that's a big, big assumption.
I'm taking some liberty, but a recent email from a parent serves as a good illustration, too. I am reworking this slightly:
"Trying to imagine my son's thoughts, I tear myself to pieces over what must be going on in there. I worry that he might be experiencing great emotional pain. I keep asking him, and he assures me he is not."
There you have it. Trust the autistic or the introvert (or both) if he or she is content being alone. You can be alone, and not lonely.
The problem for parents it that most people do define success and happiness via relationships (and a job). My best relationships are with my wife, my sister, and my mother…