If you don't want him showing up you should block him.
If you don't want him showing up on chat you can also disable chat for specific users.
I'm not sure exactly why he is showing up exactly where but the bottom line is that FB generally uses algorithms to prioritize people to whom you are more closely connected, and being a former boyfriend it is natural that they would have signals that you and he are closely connected.
If you do not want them to show him to you you need to send them a strong signal (i.e. blocking him) and they will stop doing so.