OK, the EBV/Mono test is something I have come to know a lot about. The majority of people that have it never know that is what they have. Most of the time they are sick with what they think is a "bug" or the flu for a couple of days and it is all over. Although it is easiest to get when kissing or drinking after someone, you can also get it if someone coughs hard and their saliva sprays.
As for the MCV and MCH the answer is no, they are not always high when the RBC is high. The MCV actually tells about the size of the blood cells. When they are low what that is telling you is that the red blood cells are smaller than normal. The MCH being low tells you that the amount of hemoglobin in each cell is less than normal. There are many reasons for this but one thing you need to ask the doctor is if your son is anemic. Pale, small red blood cells is often a sign of that.
Hope this helped a little. Let me know if I need to be more clear on something