If you have been treating his symptoms for 5 consecutive days and the headache keeps coming back or isn't responding to medication then I would take him in to see his pediatrician. It could be anything from an virus to an injury to an environmental irritant to who knows what.
As far as the glasses go, an eyestrain headache should have gone away on it's own once he'd gotten a good nights sleep and started wearing them again. Unless his prescription is out of date, but that isn't something that would pop up all of a sudden.
My daughter started having migraines at almost 6. We never have managed to isolate what exactly is causing them, but ever since the first one (that scared the hell out of me) she's had headaches much more frequently than any other kid her age that I know. We've gotten to the point where she now recognizes the signs that they are coming on and she can tell me or another adult before they get bad. As long as she takes some Motrin and we get her into a calm quiet environment for a while we can usually head them off.