I would really talk to his parents and also see about spending some time with both kids when one or both parents are there. That way, even if you've babysat for him before, he has more exposure to you in what may be a slightly more comfortable environment for him (when his parents around).
There's such a large variation in how autism presents that other than finding out what their family does as far as rules and what to do if he has a meltdown or something else comes up, it's not so easy to give general advice. His parents should also have an idea of what things may trigger meltdowns so you know what to avoid. Also, of course, make sure you have contact information for his parents in case something serious happens (this is so much easier now with most people having cell phones than it was when I was babysitting kids regularly!).
