He is in a modern, Western country with resources the average guy would have. No medical labs, but he can get to a hardware store. It must be non-fatal and non-disfiguring, since he's trying to keep it a secret. I've searched "accidents that damage sense of smell", "causes of anosmia", "smell disorder causes", and rearranged those terms appropriately.
Can he snuff up some kind of chemical without killing himself? Jam something up there? Head trauma seems the closest thing I've found but I can't see how he could be sure it would get the desired result. Help.