It would be important to know if the pneumonias have always occurred in the same portion of lung; if so, that would suggest the existence of some type of congenital lung structure abnormality, with poor mucous drainage/clearance, that has predisposed him to recurrent infection. I suspect that his pediatrician has already checked for lung diseases such as CF, bronchomalacia, bronchopulmonary dysplasia, bronchiectasis and for inherited and acquired immunodeficiency states. If not, that should be done.
And, yes, it is definitely time to re-visit the pediatric pulmonary specialist. It is important to identify the cause(s) of these recurrent infections that can damage his lungs. Your son may require a variety of tests, including a CT scan of the lungs and possibly tests of what is called mucociliary clearance.
If your doc hasn't run this test yet I would demand he check him for cystic fibrosis (sweat test and/or gene). Also, I would demand a referral to a respiratory doc. As you can see I'm pretty demanding 8^)