Pre-schoolers especially who attend the day care are prone to catch up infections very often. Upto 6 to 8 episodes of upper respiratory infections per year is common.
Just be sure that he gets a healthy diet- that includes lot of fruits, vegetables, nuts etc.
Avoid junk foods.
If his growth parameters - height and weight are normal for his age, you need not worry. He will outgrow these problems eventually.
You may ask your Pediatrician
supplementation on a daily basis.
Hope you find my answer helpful.
Wish your son good health.