Alex doesn't want to lose him. He cares about him. He loves Neil enough to beg for a promise.

Has Dad ever done that?

Pleaded for Neil not to run off, not to get himself injured, maimed, or killed. Not to travel back into the dangerous Wastes because James genuinely desired his well-being and not because he needed his kid's help. Didn't he once send Neil into the crossfires of a turret before? Tried to seal him in a vault where nearly every security officer wanted to bash his skull in?

Neil isn't even Alex's blood, and he is being asked to stay close. Stay alive. And the shit's real enough that Neil can't look at the other man anymore.

"Okay," he gasps. "Promise."