My sons bone marrow isnt working as quickly as it should be, results from a bmb, his red/white cells and platelets are low, what causes this??

Often unknown cause: We may not be able to pin point the cause. But you need to show him to a blood specialist(Hematologist) who will be able to establish an exact diagnosis. On that basis we can estimate the prognosis and the expected course of his blood disorder/disease

Too many causes: Low red and white blood cell counts, and low platelet counts can be due to a number of different reasons. I trust the BMB was done by a hematologist. Usually they are the best doctors to ask this question of. But to answer your question directly, exposures to certain medications, or toxins, infections, or even hematoproliferative disorders. It would be best to see your hematologist again.