When I started working at a daycare, I was sick every month for a year. The nice part of it is, after so many years of being exposed to every families germs, I rarely get sick.
My favorite ways to stay healthy:
Eat good food. The more whole, fresh foods, the closer to the ground (less-processed), the better. When you feel run down or on the cusp of a cold, add in some garlic to your food if you can. (or if you don't like the taste, garlic capsules...)
Lots of water. Regular bedtimes. Limit caffeine and sugar. Sugar feeds viruses and bacteria, so if you are feeling run down, cut the sugar and caffeine immediately.
Handwashing, and NOT with antibacterial soap. (Which usually contains triclosan, and can actually contribute to us getting sicker because it helps to create more disease-resistant bacteria). Regular soap and water and drying well is plenty. I've actually witnessed the fact that classrooms which used 'a squirt' of anti-bac instead of handwashing got much sicker than the classrooms which offered handwashing-only with regular soap. If your son's daycare uses antibacterial soap, it's time to get them some good information on why it's not the best thing in the world. (We only use it when there's no water available, and even then we follow-up with wiping it off our hands with baby wipes.)
Smart use of antibiotics are always a must.
My dad and husband swear by a 500 mg dose of vitamin c every 3-5 hours.
I like herbal tea. Do a little online research; some teas (like licorice) make a cold easier to deal with by opening up the sinuses.
My husband also uses a Neti pot for his allergies and they're highly recommended as a way to keep the sinuses moist and doing their business: fighting off the bugs.
Lastly, I've learned that a healthy immune system is one that does get sick from time to time. It's like a muscle-- if it doesn't get used regularly, then it won't work as well for you when you need it.
Good luck!