What Causes Repeated Flu Symptoms In A Kid?

Over the past two and a half months my son (3 years old), has been getting flu like symptoms then getting better for a week. Then sick again. This is now the fourth time he is sick. Fever is 103.8. Runny nose. Some coughing. And this time he is very lethargic. What is going on with him? I ve taken him to urgent care a few times in the last time he has gotten this sick. They put him on different meds. Last time he was on tamiflu and then azithromycin. Should I take him to the hospital to ask them to do blood work?