My boyfriend is a little too grabby

I don't really find it sexy when my boyfriend just grabs at my tits whenever he wants. When I ask him to stop, he usually thinks I'm joking. Even if I ask him more seriously, he just assumes he's grabbing too hard and does the same thing, but lighter. I don't want to seem uptight, and I certainly don't want to make him feel afraid to touch me, but I also want to feel like I have full control over when and how I'm being touched. Does anyone know how I should approach this situation?