Hi,
This is indeed unusual. A large lung lesion developed in the interval between an X ray and the subsequent confirmatory CT scan. Although a CT scan is much more sensitive than an X ray, but as you said, a large lesion should have been picked up by an X ray.
One possible explanation, as you have rightly deduced, is that he has a fulminant infection worsened by his asplenic state. The other could be an aggressive cancer.
At this time, a PET scan and a histopath (biopsy or FNAC) would provide vital information. Admission to a hospital and a course of intravenous antibiotics is strongly recommended pending further investigations.
All the best, and God Bless!
We got the results from the PET today...very faint uptake and smaller than on the CT. No lymph nodes showed uptake...just residual pneumonia!