Thank you for the question!
All of the above replies to your post are good tips. It sounds as if you are taking all the right steps to help boost his immune system. Unfortunately, some little ones are just more prone to getting sick during the "cold and flu" season. On top of the proper diet and supplements that you are already giving him, I would also be sure that he is getting enough sleep every night -- at least 8-10 hours and also getting plenty of exercise when he is not sick. These are also crucial to help build and maintain a healthy immune system.
One product that I would also recommend when he does get sick is an all-natural herbal supplement called "Sinupret". It is available in most pharmacies. You can try giving it to him at the FIRST sign of illness, or as a supplement during the months you know he will be getting sick. It seems to work great for myself and patients that I have recommended it to.