Shingles happens when the virus you caught as chickenpox reactivates itself by travelling down the nerves; it doesn't have anything to do with re-exposure to the virus, just immunosuppression, so I wouldn't have thought it would be a problem if he's had chickenpox and gets re-exposed. Unfortunately singles can happen through immunosupression (my mother got it, but not badly).
I don't know for sure about polio, I would see if jimjams can help, she's got lots of up-to-date figures about its prevalence, but if it's short-term it's definitely worth looking into.
Hi fio