How else to boost immune system - what am I missing?

Two years ago my son now age 7 had lyme disease and prolonged use of antibiotics. Since then during winter he catches everything and anything that comes his way. I know about probiotics, he eats activia yogurt plus chewable acidopholis. I even make fresh orange juice with fresh oranges to avoid loss of nutrients from pasteurization. This child doesn't like poor quality foods like candy or junk foo, he'd eat a bowl of brocolli or spinach instead of a doughnut or cookie. We avoid refined carbs and sugars from sweets to bring his resistance down. Just fruit sugars he recieves and carbs like oatmeal or cheerios for bfast. He likes chicken broth and has that alot the moment the sniffle or sneeze starts. We also do echinacea tea and honey. He feels fine and acts great but still he is sick every other week with something else. He is never exposed to any smoke or household chemicals. He has no asthma or allergies. I am an extremist when it comes to good air quality in the house. Another new head cold with watery eyes and sneezes started today with a fever of 102 after a healthy week, two weeks ago he just got over pneumonia, he has had pink eye, and a few other colds. The Doctors say its the time of year and kids get everything at a young age like this but does anyone else think it means his immune system is still weak from lyme and the antibiotics? He never ever had any illness as a baby, maybe one cold if any or til about age 5 which is when he had the lyme disease so something happened about that time unless it really is as simple as school aged children? I don't think anyone else misses this much school though.