I think it is an age thing but I don't think it would hurt to tell him you do not want him to put it there (he has to learn that sometime, it isn't going to be "intuitive" :-) I think it is sort of like with my grandson biting his mother's breast and her anger/"No!" behavior and moving away, etc. If he puts his hand on your breast you have to tell him "No" and if he persists (at that time) then he cannot be in contact at all with you (at that time). It's a boundary issue; you are allowed to determine who touches you, how, no matter what their age!