Has he always done it Elephant, or is it relatively recent? Is it worse when he's tired or stressed at all?
My ds has a facial tic where he scrunches his eyes closed repeatedly. It only tends to appear when he's tired, stressed or anxious. He has a couple of other tics and tells me it starts to hurt if he tries not to do it. I think this is a build up of the sensory overload that the tic relieves, the action of the tic relieves the stress for while, then it builds up again.
He also developed a problem where he was unable to read for a while because he was seeing hundreds of spots in front of his eyes and it made it a huge effort and painful to see clearly. We ended up at the Eye A&E department with him, where his eyes were checked and passed as normal. Fortunately, we saw a doctor who had seen it before in another child with AS. It turned out he was actually so visually sensitive that he was seeing the blood cells pumping through the vein/artery in front of his lens. It took a few months, but eventually he learned to compensate for it and it only happens when he's overtired these days.
If you are anywhere near Birmingham you could get him checked at Aston University Vision Sciences Department, which is much cheaper than going to a private behavioural optometrist, but obviously takes a bit longer because its students working with qualified optometrists so everything has to be done then checked. Its on my list to get ds1 to them. [http://www1.aston.ac.uk/lhs/health-clinics/optometry/ Aston University Vision Sciences]] You contact them, they send you a questionnaire to return which helps them work out whether or not they are likely to be able to help and then they contact you to book an appointment, I think.
Alternatively this website explains a bit more about how behavioural optometry might help and you can search for a private Behavioural Optometrist in your area.