Bf won’t block ex

A few weeks ago my boyfriends ex (whom he lied about when we first started dating) decided to send him a friend request on instagram. But of course.... he accepts and proceeds to follow her back.

This may have been okay if we didn't have issues of him lying to me about her presence in his life before.

I asked him to unfollow her and he did but she is still following him, and it bothers me. I've expressed this to him and he keeps trying to reassure me, nothing is or will going on.

Should I break up with him? Again, this would be okay if there wasn't a history of issues with her. He is seemingly disregarded that part and thinks i'm trying to "bully" him to doing what I want.