Can taking in too much (whey) protein elevate prolactin levels and lead to nipple discharge in a young adult male? Still no diagnosis for my son.

Probably not. High prolactin levels leading to nipple discharge have many causes. The most common causes are medications, tumors, and medical problems such as chest wall injury (stimulates prolactin secretion locally) or renal failure (decreases the clearance of prolactin.) whey is not known to increase prolactin. Your doctor should consider medication review, basic labs, and mri.
No. Have him see an endocrinologist. There are over 40 reasons for nipple discharges, some of which are true milk production secondary to elevated prolactin causing true galactorhhea (breast milk).