It is a tough one because everyone is affected so differently with this.  Tobyj is right, getting to know him will help.  You may also want to avoid eye contact.  Looking directly into an Autistic's eyes really bothers some of them.  I have read that they can find it physically painful.  Others say it causes other various symptoms, like halucinations.  Not many Autistic's can talk about their experiences, but a few have been able to. 

He's a single dad and really does a wonderful job with his son.  I was wondering if there is a specific way I can communicate with his child without upsetting him.  He is a sweet kid but he is not high functioning.  He closes down pretty quickly when I try to approach him.  I would love to help my friend by being able to babysit his son but I don't want to do something wrong.