Re: autism spectrumI have been thinking about your question. Given that you have 'new' medical information about your son, you might ask your Registrar about getting a new medical review for your son to determine in advance if he could get a world-wide clearance. That would provide some clarity.
Another factor is that your son does not need a world-wide clearance; only the employee is required to have a class 1. That would mean that he could not go to some posts with you if you were assigned to one. The likelihood of that type of assignment over a career would be high given the requirement for service in a hardship post. The options then are residential schooling, which some Foreign Service employees choose, or your family taking separate maintenance and remaining in the States while you were posted elsewhere for that specific assignment.
I hope others who have children on the autism spectrum will weigh in with their experiences for you.