What food should I give my baby to strengthen his immune system?

My 9 months old gets sick a lot. He almost always has the flu and his pediatric keeps prescribing him antibiotic and sometimes antibiotic injection when he is really sick with throat infection, and cough. Are there any kinds of food that can help strengthen his immune system so he doesnt get sick that much. I'm very clean with him and I make sure everything is sanitized, yet he gets sick too much.