During intercourse, my boyfriend's penis got a laceration and started to bleed. how serious is this?

Blood from the penis: I think its pretty safe to say that blood coming from any part of the penis is a bad thing, sans infantile circumcision. You do not provide enough information to answer your issue. Other concerns are the exposure to blood during intercourse. I would think that your boyfriend should be examined by a clinician.

Might be important : What caused the laceration? The real answer requires more information and a physical examination.