Um, my nephew is autistic and depending on how severe, it may not be a good idea to show him exactly how to shave his head. He could possibly try to imitate what he sees and severely injure himself.
I am guessing his parents have explained a lot about autism, but most autistic kids like to feel with their hands and also their lips. Just the fact that he recognizes you and associates with you is a major accomplishment.
Depending on how old he is you may want to write it down for him, but he most likely does know what you are telling him. You won't get a typical expression showing he understands, but they do.
I do want to say thanks for not being naive and relating to him, you would be surprised how nasty some people can be. I am sure his parents really appreciate it.