How do I tell him?

Sometimes I really don't want to be touched (like my boobs and butt) but my boyfriend loves playing with my boobs. I tell him no and if I say it enough times he won't do it but I wish he would take no as an answer the first time. And he won't do it until I tell him it's ok so it's not like he's doing anything without consent. I just really don't want to be touched in that way sometimes. How do I tell him without him asking over and over again?