“How can I make my child's immune system stronger?”

Good nutrition is very important. Kids should have 2 protein servings, 2-3 fruit servings, 2-3 vegetables servings and 3 milk servings per day. My experience is that kids do not drink enough milk and only eat fruits and vegetables 2-3 per WEEK not per day. A multivitamin can help if your child does not eat enough fruits or vegetables. VitaminC has great anti-oxidant properties to help fight infections. In combination with echinacea (an herbal immune booster), called vit C/ echinacea, it is very helpful for fighting infections. It comes in a chewable. The last thing I recommend is a probiotic. Probiotics do so much for gut health, including increased immunity to infections of the gastrointestinal tract, I think everyone should take one. For kids, there is culturelle in a powder or chewable or Align Jr in a chewable.
Also remember good hand washing, especially after using the restroom and before eating. When he has a respiratory infection, he needs to cough into his elbow, throw away his own tissues, and wash his hands more frequently to prevent spread. Family members should do the same to help prevent his exposure. Bring current on all vaccinations and getting a yearly flu shot are also beneficial