cvivil,
I understand how shocking and confusing this all is. It's a situation many of us recognize. It begins with symptoms that don't seem life-threatening (bruising), then the realization that something serious is wrong, then the tests and constant worry while you try to get a firm diagnosis. Getting that diagnosis is the key right now.
Normally cellularity for kids under 10 is closer to 80% than 40%, higher for toddlers, and around 60% for young adults. Having 10% cellularity (a condition called hypocellularity) means not having enough immature blood-forming cells, which are the cells that mature into active blood cells. Stem cells are the first stage for red blood cells, white blood cells, and platelets, so having too few raw materials explains why all 3 blood lines are abnormally low.
A low red blood cell count is called anemia. A low white blood cell count is called neutropenia. A low platelet count is called thrombocytopenia. When you have more than one of these conditions the refer to "multiple cytopenias". If it's all three they say "pancytopenia". ("Pan" means "all".) None of them are the diagnosis you need; they are symptoms.
Did your son have mononucleosis or was he merely exposed to someone with mono? There have been cases where aplastic anemia followed and was seemingly a result of an active mono infection. Counter-intuitively, it might be good news if the cause was mono, because the prognosis for a fast and complete recovery can be higher than when the aplastic anemia cause is unknown. But perhaps he doesn't have aplastic anemia at all.
You are correct about "megaloblasts". They are unusually large red blood cells, and their presence is one reason to have vitamin B12 and folic acid levels checked.
One of your closest major treatment center is the
University of New Mexico. There are
other major treatment centers in Colorado, Arizona, and Texas, with one of the best (and most expensive; check your insurance) being the
MD Anderson Cancer Center in Houston.
Pediatric aplastic anemia patients often get a treatment called ATG (for anti-thymocyte globulin). It's too soon to know what treatment your son needs, but you should be familiar with the name since you might hear it mentioned.
Your son will probably need transfusions until the cause is determined and appropriate treatment can be started. Good luck with this scary process. Feel free to use these forums to ask questions and find support.