It sounds like you have made up your mind that you want to end it? I think the best approach is always to be honest, though it is often the hardest approach. So if you are feeling like it just isn't working for you then just say that. I would anticipate that he would be surprised? And maybe ask some questions or try and talk you out of it? Not sure, but going in you probably want to be "prepared" for his response.

I personally feel in person is best, but if you feel safer or the need to do it on the phone that works also. I don't know all the dynamics that would help to make the decision about in person vs. the phone. Good luck with whatever you decide.