I don't post much, but just wanted to weigh in on the running part. My son is on the spectrum and is a "runner", and is insanely fast. When he runs it is because of a fight or flight sensation that he feels. He will basically glaze over and take off, not caring what or who is in his way. There have been many teachers and administrators that have not been able to catch him. He can also scale a fence like his life depends on it. But with my son, it is usually something that sets him off or upsets him. On a normal day he is not a very fast runner and could easily be caught by an adult, but if he is in a "meltdown" that is a whole other story - it's almost comparable to "super-human strength".
I of course don't know much about this little boy, but if he is anything like my son then I could totally see how he could easily get away from an adult. Hoping he is found soon, and found safe!