I think ultimately you should be honest. I don't think there is a "right things to say and not to say" list, everyone is different but if I knew someone prior to getting sick (or in the very early stages) and they were one way I would be disappoint if they were someone else towards the end.

Maybe move away from it having to be "fight it" or "you're stuffed" and more about.....(trying to think of the right words)...I'm not sure how to word it but maybe just listen to him, you don't need to respond in those kinds of way. Tell him you think this sucks and you can understand he's tired and wants it to end.

Does he seek you out for contact or is it you contacting him? I'm not trying to be nasty but do you think maybe he really doesn't want anything from you at this point? Maybe you should listen to that, maybe give him some space. What's the longest you have gone without contact since it got really bad? Maybe go a week or so and then touch base with "still here, still thinking about you and how much this sucks, if you need anything I'm always here" or something along those lines.

I know you want to be there for him, I know you want to support him but if he really doesn't want you around I also think you need to respect that. Maybe seeing you suffer is more pain for him?