This is going to sound possibly insane but do you have a similar old pair of glasses from a prior prescription you could give him?
While I can't remember if I tried this with glasses, generally giving Peanut her own thing really helped her work through her obsession and by it being "allowed" as a toy made the real one more boring since it wasn't forbidden and special. At the very least it doesn't make her obsession any worse and encourages her to obsess over the one that I'm not using.
Also if you have any tiny screws on your glasses check them every once and a while to make sure they are still tight.
Mine would always pop out at the worst possible times, Peanut loved loosening them gradually over the space of several months and they would always, without exception, fail while I was away from home. I'd have to feel for where it fell, in public and half blind, then try to screw it back in with my bare hands.