I'm going to issue an ultimatum.
Is there a reason you want to set an ultimatum?
I don't want to see him anymore, but I never told him about it
You shouldn't give someone an ultimatum no matter how you are feeling. If you think that someone like this is needed then you should do your best to try and talk to that person first about the situation.
I'm going to give him an ultimatum anyways and then dump him.
That's a complete waste of time, just dump him.