My Poor Little Boy

Currently my 5 month old infant son is sick with the flu. He has a fever and I am struggling to get him to drink fluids. I have been switching between Baby Tylenol and Advil for the fever. Advil seemed to produce better results in getting his fever to go down. I have also been giving Chase tepid bathes. Of the short five months of his life, Chase has been sick at least once each month with a cold or the flu. The doctor told me to give him Vitamin D drops because I can't breastfeed him but what I really want to know is why my son keeps getting sick and what I can do to prevent it.