Too rough in bed?

i'm kind of with this guy...we've known eachother like 4 years now and we're pretty close.we've dated off and on but are not "together" right now(it's better this way). i'm VERY attracted to him, we have amaizing chemistry, but there are some nights when he gets carried away. i like it when he's being sweet and sensual...but some nights he likes to play . the other night he scared the crap out of me...i thought he was in some kind of frenzy. i told him to stop because he was REALLY hurting me. he was pulling me by the hair and flipping me over the way he wanted and finnally, after i managed to pin his hands down...he realized he went too far and started being all sweet and cuddly. this thing he does makes me want to stop being physical with him...but i can't...we always wind up wanting eachother again. how can i stop him from being so rough with me?