I hope he is doing better.
An elevated platelet count is called thrombocytosis. Causes include anemia, early CML (chronic mylegenus leukemia), recent spleen removal, a condition called polycythemia vera, or certain other blood or bone marrow cancers.
In investigating the cause Doctors will look at other lab results such as the white blood cell count, red blood cell count, and hemoglobin as well as other components of a CBC (Complete Blood Count). They would want to know any maedicaitons being used as well. They may also do a bone marrow biopsy to see if the bone marrow is the cause of the elevated platelets or if the cause is after the bone marrow stage of blood production.
Normal platelet levels are 150-400 but can vary slightly depending on the lab and what test they use.
I hope by now he is out and is feeling better.