21 year old son has been fainting with eyes wide open and set. (very scary) He had stage 3 melanomia 2 years ago. Cemo caused pituitary gland to swell?

Neurologist. He may be having atonic seizures and there would be concern about metastases to brain. It is urgent that he be seen by a neurologist for these spells for diagnosis and treatment. The neurologist would work in concert with the oncologist.
Get this diagnosed. Based upon information provided, I suspect he is having seizures and likely should be placed on anti-epileptic medications. Contact the doctors, and likely have a neurologist get involved.