Productive cough in your son may be due to bronchitis
. Nebulisation helps only when there is bronchospasm
. Cough may also be a variant of asthma
in which bronchospasm will be there. But for making a diagnosis of asthma eosinophil count
and pulmonary function test will help.
Sodabicarb is not used for nebulisation.
So I would suggest you to consult your doctor to find the cause of cough by clinical and some lab tests and then management accordingly.