Some things can affect the number of colds that kids get. Kids who are around other children (school, day care, siblings) get more colds than those who aren't. This doesn't mean we should stop sending our kids to school and day care, but there are some things we can do to try to limit their exposure to germs.
Colds are spread through contact with nasal and mouth secretions. Have your child wash his hands regularly, as that is the best way to prevent the spread of these germs. He should wash his hands before every meal, and when coming in from playing outdoors.
Children who are exposed to cigarette smoke also get more frequent colds. If there are smokers in your household, they should always smoke outside of the house (better yet, they should quit). It is also important that your son get enough sleep each night. Studies suggest that sleep deprivation increases susceptibility to colds.
People used to think that taking large doses of vitamin C could help prevent colds, but studies have shown otherwise. If your child has unusual dietary habits and could be lacking certain vitamins, then it would be reasonable to give him a supplement, but there is no known benefit to giving a healthy child extra doses of vitamins.
Recently, there have been claims that certain herbs may help prevent colds, but since these are still inconclusive, I don't recommend this approach.