Originally Answered: How to boost my 2 1/2 year olds immune system?
Make sure he's getting enough sleep. My kids (ages 3, 6, and 7) have not really been sick this winter at all other than a couple very minor colds, and I'm convinced that it's partially due to me being so strict about bedtime. (And I'm not usually so strict over the summer, but I have been this winter -- lights out by 8:30 for the older two, 8:00 for the 3-year-old, and they get up anywhere between 7-8am.)
Also, offer a daily multivitamin. (My kids like the gummy ones.) Even if you decide not to give it year-round, it could really help this winter.
Finally, try pushing some different fruits/veggies. Even though berries are usually best in summer, we've found that the blueberries are decent right now, and they're good for keeping your immunity up. Also, try oranges or clementines.
At the very first sign of illness, start trying to get him some more sleep, and serve soup with lunch or dinner. We've done that with the kids this winter, too, and it has seemed to help a lot. I personally take extra Vitamin C when I feel a cold coming on, but I don't give any to the kids (except maybe in the form of a little cup of juice).
Some of this he'll just outgrow as he gets exposed to more illnesses, but you can try to take some preventative measures in the meantime.