During sex my boyfriend thought he was going to ejaculate and he was bleeding instead. why is that?

He might have blood in semen (hematospermia) so the semen may resemble blood. By UPTODATE: Hematospermia, the presence of grossly bloody or blood-tinged penile (seminal) ejaculate, is an uncommon condition. While it is usually alarming to patients, the cause is almost always benign.

There can be many reasons- from local trauma to conditions of the prostate and genital tract- need investigation