thanks for your query at HCM
there seem to allergy
happening to your son because he is developing nasal blockage , cold
and phlegm formation.
first you have to identify the disease or allergen that is causing the problem every alternative month, other wise there is no use in treating him every month.
for nasal blockage use nasoclear drops taht will releave nasal blockage
there are several mucolytic agent
available for nebulization that will liquify phlegm.
they can be used only during the episode not for routine use.chest physiotherapy
can also help in clearing phlegm.
hope this will help you