Unfortunately, there's really no evidence that taking extra vitamin C will help prevent getting a cold or other infections - not that I'm advising against vitamins, but just that medical research doesn't show a definitive cause/effect relationship.
Different people have different immune systems. Why has our 2 year old daughter had 10 ear infections, and our almost 4 year-old son had 1 in his life? Different biologies despite being in almost exactly the same environment (different classes at day care) and the same parents.
When I was going through chemo, I basically had NO immune system and worked in pharmaceutical sales going in and out of doctor's offices all day, every day. When I asked the Oncologist if I needed to take a medical leave to protect myself (infection could have seriously killed me), they said to be more diligent about washing my hands and using hand sanitizer. I had 2 kids in day care bringing all kinds of things home and was lucky to avoid infection.
I'm sure a physician, nurse, pharmacist would give the same advice. Products that claim to boost immunity have been investigated by the FDA/Federal Trade Commission lately as false and misleading.
Hopefully, the worst of it has passed, and he'll be better soon.