Building baby’s immune system.. help!

Building immune systems.. HELP! So my 13 month old has had a hard time adjusting immune system wise back to daycare. It doesn’t help that they also moved him to the next room up. He gets frozen BM, but any suggestions on how I can boost his immune system naturally? I feel like every few weeks he is sick or has a low grade fever.

(He was not in daycare and had limited exposure to anyone outside our home from mid March to end of July because I’m a teacher)