My 16 yo old son had a blood draw to check for anemia and only abnormality was low wbc. normal 4.5, his was 4.2. two weeks later his wbc is now 3.2. slightly high monocytes. neg for mono. not sure what is going on. scared its some type of cancer?

Fear not: Most likely it is simply a common virus which has mildly, temporarily suppressed his white count, assuming the red count and platelet counts are normal. It sometimes takes weeks for the white count to bounce back up. Not to worry. Cancer very unlikely if all else normal. Follow counts every 1-2 weeks till normalizes. Listen to your doctor. They will refer you to hematology if needed.