Why Does My Toddler Have Frequent Cold, Cough, Fever, And Need Regular Nebulization Despite Past Lymph Node Surgery?

I have a 3.2 year old son He has suffered from health issues since childhood. He had lymph nodes for which he had been operated as puss was formed in it and it was enlarged. He suffers from frequent cold cough and fever. He has congestion and needs to be nebulized frequently.