What Causes Bleeding On Sucking Nipples?

I'm 19 years old and when my boyfriend tries to give me I hickey or even suck on nipples, I bleed. He does not suck hard he lightly sucks n I bleed. It stops but his mouth is always filled with blood.he tries to spit it out right away. Why does this happen? What could be causing it? What can we do about it? Is it harmful for him to get blood in his mouth?