[NoobArcher]

As everyone has said, autism is a HUGE spectrum, and it actually umbrella's over a lot of other less well known syndromes like Aspergers and the suchlike. having worked with a lot of these kids, my suggestions would be as follows.
1. Keep it structured. These kids thrive on structure and boundaries. It might even help to make a little list when he draws up for example, and to get him to help you make it, then he and you repeat it every time. Its a little like dog training in that respect, even though i know it sounds cruel. Repeat the behaviour again and agian until its second nature.
2. Ignore bad or unwanted behaviours. Autistic kids will go into themselves and you'll lose all rapport you had with them if you shout or reprimand them in a way they are unused to. The parents will be able to help you with this.
3. Don't be surprised if they are unable to look you in the eye or appear to not be listening. They will be.
4. Physical contact from someone they are unfamiliar with often makes them nervy and uncomfortable. If you want to touch them to emphasize a teaching point, tell them that you're going to.....' ie, I want to put my hand on your hand to show you something, is that ok?'
5. Don't be surprised if they are very fidgity, or have a tic. If the equipment round you is untidy or disorderly, chances are they will want to rectify it, and its best to let them, as they won;t be able to concentrate until its done.

What I've written sounds a bit extreme I know, and its perfectly possible that the child you're going to teach is nowhere near as acute as what I might have described. They'll be diddicult to teach in a group environment though, especially initially, as they'll need constant reassurance and guidance. All I can say, is to persevere with it, as you'll both get so much out of it. You wait till you have that moment when you know you've gotten through and really made a connection....its awesome.