I second having your son get checked for Neurological and Allergy related problems. The other adults I have talked to who had their SPD become a problem as an adult usually had some other health problem or medication for that problem that ended up triggering the increased sensitivity. I have an Ion Channel disorder that causes a lot of stress on the body. One of the medications for that disorder that I took for a few years did actually have increased light sensitivity as a potential side effect.

I got my Irlen Lenses from the Irlen Center. http://irlen.com/index.php

Hopefully you can find your local Irlen Center and check them out. The initial tests are pretty simple and are close to affordable. Your son will know by the end of the tests if this is the right track for him to look further into it. If it does look like the Irlen Lenses might help him, they will start him out with an overlay to go over anything he reads for two weeks. If he isn't suffering from as much eye strain and his reading is improved in those two weeks he should go back for further testing. This time the testing is with the lenses themselves. I took the lenses that seemed to help me the most for test walks about 10 feet. The difference between the right lens, wrong lens, and my standard vision was quite great. Buying the lenses was very expensive (just under $300), but it has helped me so much that it is worth it.

I wear prescription glasses and the $300 was to tint the glasses so that the wavelengths of light that were irritating my brain couldn't get through. It is possible to get Irlen Contacts, but that is even more expensive and only if you are wealthy is the vanity bonus worth it.