Boasting immune system
I know it's weird to say this. But it's a fact that the only way to boast immune system is to be contacted/infected with the bugs. That's how antibodies and other body defence systems work. Assuming that the kid are eating well, getting enough nutrient, having enough rest/sleep, enough fluid intake, exercise frequently, then, exposure to the bugs is the only way to boast immunity.
Supplements only help if the kid is lacking a certain mineral/vitamin/malnutrition. If the kid is not lacking of any of the above, supplements won't help boasting immunity. But of course, no harm giving if you don't mind giving that, but make sure the dose is right, as overdose of vitamin/mineral is not necessarily a good thing.
When the kid is sick, ask the doctor enough questions on what's the sickness is about, monitor your kid closely and know that he is not lethargy, then give him a chance to fight the bug on his own, without any med intervention. That's the most effective way of boasting immune system.
In a brouchure that I got from the PD clinic, it's normal for young kids to fall sick (flu, cough, etc) up to 8 times a year, until they are older and immunity has built up.