My son (7 years old) is coughing a lot for a very long time now. Any alternative treatment you could recommend? Any food we should avoid?

Question. We need a little more details here. How long has he been coughing, does he have a fever, and is he having trouble breathing? What have you tried for the cough?
Allergies/asthma. The most likely culprits are allergy or asthma. I recommend you take him in to his doctor to find the cause of the cough, as the treatment varies with the cause. If it is allergies, then there are a few herbs that can be helpful - bromelain, nettles, and quercetin are a few.