I think you have to go to the Autism Forum to access the relevant Health Page.
If he has sensory problems he may not be feeling the sensation as we would and when he 'hurts' himself that could be the time he finally 'feels' his body parts. That would be called hypo sensitive. When there are sensory differences the perception through the senses can be hyper or hypo and can fluctuate between the two throughout the day and day by day. So you won't necesarily get the same response to the same stimulus. My son is on the autistic spectrum, and when he was younger he would show some bizarre repetitive behaviour, for example repeatedly jumping into the swimming baths wearing his armbands. He would swallow alot of water, and even vomit, but would go right back to jumping in again and again. The only explanation I had at that time was that the jumping in the swimming pool gave him a sensory feedback of his body entering the water.
Although you don't say your son is on the spectrum, it might be useful for you to google the name Olga Bogdashina and read an article by her in Autism Today. If it sounds relevant she has a book called Sensory and Perceptual Differences in Autism and Aspergers. At the back of this book is a questionnaire that you can complete that will give you a sensory profile of your child. This woman is reputable and her questionnaire is used by our autism department. She was also invited to speak to professionals and parents in our city.
Although you can have sensory issues without autism. Most, if not all people on the spectrum have sensory processing problems which range from mild to severe. So it might be worth considering whether autism is a factor or not. Delays in speech and social communication/interaction are part of the diagnostic criteria. And if autism does play any part, then you will need your child to have access to autism friendly supports/ approaches/strategies etc.
At the top right hand side of this page you can click on the Health Pages link and read the Diagnostic Criteria for autism. Some people have posted examples of the behaviours behind the characteristics which is helpful
Has your son been evaluated by a specialist yet? With some conditions the stimulation from the pain, although negative, is fascinating. They will repeat it because it is new and interesting. Of course you need to stop him from hurting himself, even though it upsets him. Kp's suggestion of a developmental psych eval is excellent.
Hang in there and keep us posted.
Speaking to a developmental psychologist would be a good course of action.