It is expected to have a high atypical lymph count in a vital infection. He'll be very tired and might have an enlarged liver also. There are different viruses that cause the atypical lymphs to go up fast (the reactive atypical lymphs are actively fighting a viral infection)... The flu (influenza A or B) cause it as well as mono and CMV. If the Mono is positive, many places do a titre just to see how positive... That can delay test results for a day. Rest, rest, rest and plenty of fluids. It'll seem like he's sleeping a lot but his body is fighting a battle. I pray you here something soon... Let us know.
It sounds like your son may have infectious mononucleosis. Does he have a swollen lymph nodes in the neck too, and fever?
Atypical Lymphocytes or Reactive lymphocytes are very characteristic of Mononucleosis (Eptein Barr Virus) specially when you see that high count. The best way to confirm it is by the Mono test. If this is the case don"t worry to much, your son will be fine in couple of days. And more important tell your son not to kiss anybody during this time.
If he had a fever and swollen lymph nodes you could have him tested with a mono spot test. He should lots of rest.