It is called post-herpetic neuralgia, runs months up to a year depends on individual's onset of the shingle, status of vaccination, immune system etc, usually treated with TCAs (amitriptyline) or anti-convulsant (gabapentin). consider the age of your grandpa, I recommend you try capsaicin, an OTC cream use topically as directed, which may help relieve some sx, or go and see a PCP to get the prescription. Good luck, and hope your grandpa gets better soon.