Remedies/ recommendations for cold

My son had a cold basically for 6 months (pneumonia twice.. last time he had was about 3 weeks ago). He still coughing and the doctors just recommending the same thing that obviously not helping. I know it’s not much they can give him since he’s only one, but I definitely don’t want it to turn into pneumonia again.. do any have any recommendations that might help him get rid of this cold/ get the mucus out of his system? It’s like the mucus comes up, but he swallow it bout time it get in his mouth