Why does he do that is an important question. All behavior has a reason, and serves some kind of purpose. When you can figure out what the purpose is, then you can help him to find another way to get his needs met.

Some possibilities, particularly for a child with autism, are that he is getting sensory stimulation that way when he either he can't handle what he is feeling or isn't feeling. People with autism have very specific tolerance levels for all of their senses, and too much or too little stimulation (sight, sound, feel, smell, taste, and also movement and balance) may just be intolerable for them. Hurting himself may feel better to him than whatever else he is experiencing.

Communication is also extremely frustrating for people with autism. They don't 'get it' sometimes and he may be trying to show you that he is angry or that he needs something or just that he is frustrated.

He probably lacks awareness of your feelings when you see him hurt himself, as he most likely has extreme difficulty making sense of his own feelings, let alone anyone else's.

If you are interested in more information about autism resources, you can PM me. Concerning his behavior, write down what is happening before the behavior, what he does, and what happens after the behavior. Keep notes like that whenever you see the behaviors that are problems. Soon you will start to see patterns. What happens before can tell you what he might be having difficulty with, and what happens after tells you what he is needing and trying to get. Then you can help to alleviate whatever might be bothering him and teach him other ways to get what he needs.

Observations like that also work for people who don't have autism. I work with people with behavior problems (mostly developmentally disabled - some autistic, some mentally retarded, etc.), and this is the method that I use with them. My homework for my own therapy is to apply that method to my own problem behaviors (including self-injury) and address my problems the same way I would deal with them in my clients.