I take a high dose multi vitamin (Life Extension Two-Per-Day capsules), without Iron, as too much Iron if you are not deficient is a dodgy ingredient to have accumulating in your body . I vary rarely get a really bad cold and contribute part of my wellness to 1000mg+ of daily Vitamin C tablets (Vitamin C is water based which means your body can't store it, excess vitamin C will be expelled in urine, if you overdose you will get a bout of diarrhoea). You get Vitamin C from fruit and veg, it likes to give colds a good punch up
just don't take the chewable variety, they rot your teeth! Vitamin C uses the same GLUT 3&4 (Glucose Transporters), so it is possible for the vitamin to give a false high reading - over 1000mg can do it my memory serves me correctly. Also, unlike a pharmaceutical drug, Vitamins take longer to work, it may be several weeks before he feels any benefits. One of the ingredients might upset stomach too.
Is it ok for him to take them? I don't know - he should consult his doctor, explaining to his doctor that he has an appalling diet that may indicate vitamin deficiencies.