Hello, I Have A Six Year Old Son. He Was

Hello,
I have a six year old son. He was born with asthma and allergies. He is constantly sick. He always has a cough and he is always congested. He will get better for a few days to a week and then he is literally coughing again. At times he gets coughing so bad he cant breathe and I have to give him his nebulizer. I bring him to the doctors and they can never tell me why he is always sick. I wonder if he has a weak immune system or something. It just gets really frustrating because it doesn't seem like I can do much to help. I don't like giving him cough meds because they are not good for such a young child. Do you have any suggestions on why he is constantly sick, all year around?