My 5yr old son has had two abnormal EEG's. It's in one area of his brain, could a tumor be the cause?

Needs evaluation. EEGs are done when one suspects seizure activity. Focal areas of seizure activity, determined by abnormal EEG may need further testing in the form of scans of the brain. You should consult a doctor/pediatric neurologist to look further into this. This is not alarming but needs further tests at least to disprove such diagnosis.