My brother has detected an ample amount of blood in his semen when he ejaculates. What could be wrong?

Hematospermia. Blood in the ejaculate is called hematospermia. It is often benign. There was a recent paper published demonstrating a link with prostate cancer. There can also be an association with infection. Despite these connections, if a doctor decides your brother has a normal feeling prostate and he is young enough that he is not at risk for prostate cancer, he is likely safe. His md should evaluate.
Likely nothing. Your brother might be masturbating too often if he is young. After prostate biopsy if of mature age. Rarely due to infections such as chlamidia, herpes or tricomonas. Stones in the prstate or prstatitis can cause blood in semen, however, he would have urinary symptoms or pain. Most cases are self limiting.