I don't know how severe your son's outbursts might be but I have a good friend who works with children that have severe Autism and often Tourettes too. He has cards printed that are basically business cards and just say something like "You are watching an Autistic child. The best way you can help is to just leave him alone.".
He'd hand these cards out to people that insited on staring and talking and according to him just giving someone a card without even talking to them explains it all at once and tends to make people just leave the situation.
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