When was the first time he developed symptoms of mono?
Toddlers and preschools usually have mild symptoms and can be sick for only a few days.
Blood tests are performed to look for an elevated number of white blood cells (lymphocytes) or abnormal-looking lymphocytes. These blood tests won't confirm mononucleosis, but they may suggest it as a possibility.
Both, leukemia and mononucleosis (and other conditions) are characterized with increased percentage of atypical lymphocytes, but doctors can distinguished atypically changed lymphocytes in mono and leukemia under the microscope.
Also, you have confirmation test (EBV testing) for the cause of your son's condition.

Yes he has tested positive for Mono on an EBV Panel. I was just concerned because all symptoms went away and have been away approximately a month, and he has started complaining about throat and headache, and has ran fever the last three days. So I was worried that it may be something else, because of the time gap between symptoms.

These symptoms could be recurrent mono symptoms, because, sometimes the disease lingers for a year or so, causing recurrent attacks that gradually become more mild.
Usually, in healthy kids, resolves completely with one attack.
My question is if your son was healthy one before symptoms of mono?

Also, one of the complications of mono is abnormal blood cell production. The virus can suppress the ability of the bone marrow to make blood cells, such as platelets and white blood cells. This can lead to bleeding problems and increased susceptibility to infection, respectively.
I guess, after the mono disease, your son got decreased immunity and increased susceptibility to infection. So, the symptoms he is experiencing now could be caused by other infect, for ex. from adenoviruses that are very frequent in infants and young children and can give the same symptoms as mono.

You can talk with his doctor and ask for some tests about his immune system's status.

Thinking of you and your son!
Well wishes to your son and your family!
Marija

blood test results from mono and leukemia would NOT be the same. i would recommend getting blood tests done for him. thats really the only way to determine. good luck!