I would suggest keeping it about yourself, and not him. Keep it simple.
For instance "This relationship isn't working for me. I am stepping away from it. I wish you the best."
Not that exactly but you see my point. It is true. It is a statement about yourself. It is clear. It is to the point.
Do you have a plan as to how you are going to leave this conversation/meting? Hopefully you can set it up so that you can get out of the situation so you don't get entangled in lengthy 'debates' or 'discussions' etc. I'm not very good at that so I usually default to just repeating myself over and over. Not sure if that is a really great tactic or not but at least I know things won't get twisted and I won't say something I have to take back, or don't really mean, etc.
Ugh - always hard conversations. I'm so glad you are looking out for yourself though!!