ANSWER - I never know what I'm going to find in my inbox but I don't think I ever expected a question like this. I cannot believe that people can expound such dangerous nonsense. Autism is a recognised medical condition, diagnosed by medical professionals. It has nothing whatsoever to do with evil spirits. The symptoms couldn't be more different. I'm not saying an infestation of a malign spirit can't happen, certainly it is possible to become possessed. But a person who was possessed would display completely different behaviours to somebody diagnosed with autism. I don't have any medical qualifications so I am reluctant to give an opinion on the outward symptoms of autism, but I can tell you that the symptoms of possession don't differ greatly from anything you will have seen in horror films. I will say it again, the two things just couldn't be more different.
This person is telling you dangerous rubbish, ignore them completely. And don't give their words a second thought, your son is most definitely not possessed.
Image http://www.sxc.hu/browse.phtml?f=view&id=618649 (Berka)