5y/o son having fainting spells. Not related to dehydration or heat exhaustion. Blood work, EKG and echo cardiogram came back good. What could be wrong?

Syncope. This is not normal, especially in a five year old. Please see a pediatric cardiologist and neurologist.
Mgt. I would definitely recommend that a neurologist and cardiologist complete a consultation. Please have your pediatrician obtain standing, seated and supine BP and pulse oximetry readings.