This is going to be really hard to say, but I am going to have to leave him at some point.
I have heard that you want to break up with your boyfriend because he has cancer, is that what you want to talk about?
He's been making me more and more sick inside. I don't know how much longer I can go on watching him suffer.
You should be honest with your boyfriend about how you feel.  But at the same time, he needs you to be there to support him, because he is feeling even worse than you are.  A trusted friend can help you to cope with your own pain during this difficult time, so you shouldn't abandon your boyfriend.
I want to leave him. I'm so over this.
If that is your final decision, you should just tell him the truth. Being honest is the best policy and stretching it out longer than need be is just going to hurt both of you even more.