Not sure about the treatment for the pneumonia and how it fits into the allergies, but.... fwiw
Our (then 11 yr old) son had pneumonia early this past spring. It came on very quickly. He was given oral and injection antibiotics to cover all the bases (he had it in both lungs). He also has suffered with "seasonal" allergies for years. We initially tried Sinulair (when he was about 7 yrs) and it gave him stomach aches,so we discontinued and tried the Nasonex spray. It seemed to help somewhat, but did not totally resolve his symptoms. And it does require regular usage to work.. not just "as needed"....
So, we began giving Allavert/Claritin daily. It was a mild help... but certainly didn't keep an investment in Kleenex from crossing my mind daily.
After the pneumonia was cleared up for our son, his doctor (recently changed doctors) suggested Zyrtec instead. I had been hesitant to use it, since the label seems to have more side effects than the loratadine (claritin/alavert), but under doctor's suggestions we gave it a try. It works wonders, compared to the other OTC meds at least. He is not completely symptom free, but it really made a big difference.
Our son is also currently undergoing allergy shot therapy to try to reduce his reaction completely so that eventually he can forego the OTC meds altogether and have a "normal" Kleenex only with a cold, kind of life.
Zyrtec also seems to be a recommended regimen for patients with asthma symptoms. You might consider giving it a shot if your son does suffer with any allergy symptoms. But the allergy symptoms and the pneumonia probably aren't related, except that allergies might weaken his bodies immune ability to fight it off. ?
hth