I Have A 5 Year Old Son Who Is Having

I have a 5 year old son who is having repeated high fevers with enlarged liver and enlarged spleen, vomiting, and headache. Doctor thought of mono right away. He was tested for mono and epsteen bar virus both of which came out negative in April. His symptoms went away and everything was great for about 3 weeks when the same symptoms showed up again in May. He has had a CBC with differential, ANA, blood culture, sed. rate and liver panel tests. All of the test's have come back normal with the exception of hemoglobin count had a slight decrease from the first round of blood work done in May. He gets better in about 5 - 6 days and his liver and spleen go back to normal size, he looks and acts better. He is going for a sonogram tomorrow to make sure nothing like a tumor is spotted.
What else could be causing these symptoms? If it were cancer would a blood test show it? Could it be mono but the first test did not show it (even thought he was really sick at the time the blood was drawn)? Are there any steps we should be taking at this point? Any other tests that we might want to consider?
Thank you!