One of my daughters had pneumonia at 9 months. They x-rayed for diagnosis only once and because we were at the ER. Does he have a fever? Is he wheezing?
My oldest daughter (9) has asthma and is on pulmicort. When she was little, she would end up on the nebulizer whenever she had a cold. If she got really bad, the doc would put her on steroids. She has never had a lung x-ray.
If your son sounds like he is breathing hard or is distressed in anyway, you should take him to see the doctor immediately.
BTW, Doctors are not Gods. You can talk to them when you disagree with their treatment course. Don't be afraid to speak up. This is your child you are advocating for. The doctor will only know your concerns if you bring them up.