Your colleague will continue like this so long as you let him.

If he really has no authority over you, there is nothing wrong with pointing that out once he crosses the line. After all, if he isn't your boss, there is nothing wrong with a little quote-unquote 'insubordination':

HIM: "Why are you doing it that way?"

YOU: "Because it works."

HIM: "When are you going to to do X?"

YOU: "That's between me and my boss."

HIM: "Don't say hopefully to me, I want a straight up answer."

YOU: "That's nice. I'll hopefully have it done in X time"

At this point, he will either realise that continuing to confront you will make him look like an idiot or he won't, and he'll make himself look like an idiot, only realising way too late. Either way, he'll probably stop soon enough to make your last few months at that position less painful.