My boyfriend cannot masturbate long without pain on his penis?

Give it a rest: Most often is from "overuse." take 3 days off from touching it other than to urinate, and if not resolved, then see a dr for exam.

Why?: Wears the pain? If it is rough masturbation, maybe he is simply causing a strain to his penis, etc. Have him check with his dr. Or a urologist if it persists. Best wishes.

Depends: Really depends on how often and whether he uses lubrication. Most common cause is overuse, need to check with your doctor.