Autism / Autism Spectrum Support Group
Autism is classified as a neurodevelopmental disorder which manifests itself in markedly abnormal social interaction, communication ability, patterns of interests, and patterns of behavior. Although the specific etiology of autism is unknown, many researchers suspect that autism results from genetically mediated vulnerabilities to environmental triggers.
http://www.cdc.gov/ncbddd/autism/signs.html
does he show any other symptoms as addressed in the link above?
Here's what I mean by that.
Autism doesn't cause kids (or grown-ups) to slap. But, acting out after something (like a scooter) fails to work as expected (or even desired), even if the failure is because he was tired and fell is consistent with autism [but also with three year olds]. An early example given to me of autism is the grizzly bear playing with the snowball. The grizzly bear crushes the snowball and is then enraged because the snowball broke.
As far as where they learn slapping, I'm not sure that it's always the same place. They see it and try it. Though I think now's the time to let him know that it's not acceptable. You'll find a lot of people who will want you to view it as a form of communication - and it may be. But my experience is that you'll still be dealing with it years later if you don't nip it in the bud now. [There is debate about the best way to do that but my suggestion - if practical - would be to send him home if /when he does it.]
I hope you find a solution that works for you and your grandson.