Hello,
Thank you for asking your query.

Sorry to hear about your son's health issues. I will try my best to help you here.

As you might be aware there can be a number of causes of enlarged lymph nodes in the neck in a child. Most common cause for this kind of sudden onset lymph node enlargement is an 'infection'. Your Doctor has did the right thing by ordering a Monospot test.  Even if the test is negative it could be number of other bacteria or virus causing the lymph node enlargement. Its prudent to consider other conditions like Kawasaki's disease. Neck node enlargement is common condition usullay managed by a Pediatrician , so please have your son evaluated by a Pediatrician. He will guide you regarding further steps to be taken. 

Leukemia would be one of my concern, and I would like to rule that out to be safe. Since your son is having headache and bleeding nails (can be a symptom of Leukemia  but can be a symptoms of more common illnesses too ) its always better to do some blood investigations to rule our Leukemia. 

I would recommend the following simple blood tests which can give more clue.

-  Hemoglobin levels

-  Total Count, Differential Count

-  S. Bilirubin

-  Peripheral smear study.

Incase the symptoms are persisting beyond a week from now, you should discuss the possibility of carrying out these tests with your Doctor. 

Hope this helps.
If you have any followup questions, please feel free to ask via this email itself.
Thank you.