'I suppose' is not exactly a vote of huge confidence, but considering the shaky ground they're standing on at this point, he will take it gladly.

Zack laughs, clapping him cheerfully on the shoulder. "Great! We're friends now. No more past tense." Maybe it'll help him think less about all the past tense stuff that ruined his life. The fact that he doesn't have to talk about anyone dying now is an added bonus.