I'm pretty ahead, but I think it's more that cliff can't see a future with Leon. Not cause he doesn't like him, or feels like a parent to him, but rather because he literally won't live that long. He sees it as him causing unnecessary pain, we'll have to wait for more chapter to come out tho
I kinda feel like cliff sees himself as Leon's replacement mother in a sort of way but obviously Leon does NOT see it that way (/TДT)/