Out of curiosity have you always suspected that something was up with him? Or just recently. What's his reaction when you correct him? Without knowing the child my first instinct would be to think that if he sees his siblings getting attention because of special needs, he could be out for a share too? Maybe? I certainly wouldn't say anything about autism because I (as in me) don't feel qualified to but I'd ask if they've noticed a change in behaviour at home and at least draw their attention to the fact that you have. It must be hard for the whole family with so much on their plate.