In the modern day I would really avoid using "autism" as a negative descriptor or analogy in published works like this, as much as I may say it in private contexts to friends.
I've always been okay with dark humor, never thought any subject (e.g. holocaust, cancer, etc) should ever be taboo matter for humor regardless of how dark it is in reality. I would never presume to tell a comedian "the material you used for that skit, that's really not something to joke about."
To me it's the same principle. I certainly would never mean to mock those with autism or their families who struggle alongside them, and you have my utmost respect for the difficulties involved, but I don't feel that the subject is of such gravity that I can't use the word as hyperbole in a private context.
Not sure it's any of your business what he says to his friends in private, even if you have an autistic son.