Thank you for your query. I can understand your concern.
As you have not mentioned the age of your son I presume he is in the 1-5 year age range. At this age it is very common for children to have repeated upper respiratory tract
infection which is usually viral in origin. If the infections persist or occur very frequently then blood tests are warranted. As the blood test has been done in your son's case let the doctor review it. Leukemia
is not so common and it is with a proper work-up such cases are picked up.
I suggest that you discuss with your doctor so that your concerns are answered.
I hope I have responded to your query satisfactorily.
Wishing your son a speedy recovery.