I actually hadn't had his hearing checked, for some reason it didn't occur to me. But I will do that too - he's never shown any obvious signs of hearing loss but I know that it can sometimes be hard to pick and it won't do any harm to have it checked.
Before finding an audiologist, it might be helpful to take a look at some info about auditory processing and see if anything looks familiar to your situation (for example:
http://www.caddac.ca/cms/page.php?166). Most audiologists only check the physical aspects of hearing. A few rare ones test auditory processing, which is how the brain translates the sounds it hears - so you can have excellent physical hearing but still be receiving a garbled signal. Auditory processing disorders are quite common, and can have dyslexia-looking impacts on phonological understanding (it's hard to make sound-letter relationships if there is interference with the sound). If, for example, your DS hears you but often seems to misunderstand what he is told, or has unusual difficulty understanding in noisier places, it might be worthwhile tracking down a audiologist who can look at processing and not just regular hearing issues.
