He finds comfort in holding your hand and when you aren't doing this he is showing that he is upset by your absence in hitting himself. I would talk to a psychiatrist about this and see what help you can get for him. I don't know how disabled he is, but maybe a stuffed animal for him to hold when you have to do other things would help. Talk to a professional since we have no way of knowing the extent of his disability. Take care.
That sounds more like a developmental disability such as autism but only a psychiatrist would be able to provide a conclusive diagnosis. The best thing to do would be to take him to a child psychiatrist so they could follow up with what would be the appropriate diagnosis and treatment.
Have you talked to his doctor? They would probably be able to help best.
You are posting this on the depression site, and I am not sure that your question relates to depression.
Would another site be more helpful?