My 16 yr old son has been referred to a pediatric hematologist for neutropenia. cbc w/diff show wbc 4.0, mpv 7.0, %mono 2.2. what do they suspect?

Depends: If junior appears well and no abnormal cells were present in the blood, leukemia is unlikely. Some people simply run a low absolute neutrophil count and they may just be covering themselves for liability. Or if the absolute count is below 1000, junior may actually be sick -- lupus, medication effect, and cyclic neutropenia are three of several possibilities.

More info!: The hematologist will be able to sort this out. You mention a very slightly low white cell count and a slightly low mean platelet volume. That isn't much information. The specialist will have a compete report and be able to obtain a thorough history and so a good physical exam. He will also be able to tell you of your child needs further testing.