You deserve a lot of credit for keeping cool and keeping this in perspective. This hasn't happened to my child but I know someone on the other side of the equation -- a child care worker who realized one kid was not there. He had left the building and was all the way down at the major intersection at the end of the very long driveway. He was OK -- the difference is, he was a preschooler.
As others say --your son so obviously needs a full-time aide. I am actually surprised he doesn't already have one due to his being nonverbal and his having a clear history of running off. Next time he might not be found so quickly. I would go, in person, to the school tomorrow -- not in a week or later but tomorrow. See the principal and say that until your son has a full-time aide at his side, you will have to attend school with him. That will basically make them freak out a bit, and I know you don't want to upset them, but though they "did exactly as they should," this should not have happened, and it should not happen again. They will not want a parent sitting in class all day so the statement "I will have to bring him here and stay here all day" should spur them to get your son an aide.
Isn't he already on some kind of formal, written IEP (individual education plan) that you have already worked out with the principal, counselors, etc.? He should have one. This is a document that requires the school to provide certain accommodations to a student. A full-time aide can be one of those accommodations. If this is public school and you do not have an IEP -- get one. It is written assurance of what the school MUST provide, a kind of contract you can insist be enforced.