Thanks all for the speedy replies. You guys are great.
Can he have picked up enough clues to make some of those connections himself, so that he takes on part of the explanation himself?
Currently no, and your question makes me realize that revising the ending so that MC does in fact make the connections himself is the very key to solving my problem. Thank you.
Also, is he just sitting there while they explain? What else is going on this scene?
Yes, he's just sitting there (standing actually, but the same, he's passive once he confronts them, so yuch).
What is the greater good, if any, he sees for the sacrifice.
The inciting incident could lead two nations to war; he believes that he must find the culprit to stave that off and comes to realize that only by making that sacrifice (specifically, swearing allegiance for life to the underworld boss) can he find whodunit. So right now the big dramatic moment is his decision to give up his own future in order to learn the answer. But once he learns the answers, the drama fizzles.
The boss knows the MC's flaws. Do you also? Let him fondle those defects, or he could go in the other direction and ask him to join the dark side. What will the MC do?
The MC's biggest flaw is intense loyalty. He comes to realize that he has confused loyalty to the nation with loyalty to his boss (the de facto ruler), and his boss has taken advantage of that.
But once he learns this, the MC basically walks away -- the peace has been preserved (his ultimate goal) but he is disillusioned that his boss brought the world to the precipice of war as a way to preserve his hold on power. So he figuratively spits on the guy, leaves and the book ends with him entering his new life, pledged to the underworld. Its not a happy ending.
One way it might work is if the MC is valuable to the boss and boss's cronies--like, he's a whiz at a part of their Sinister Plan they're having trouble with. Then they might explain what they're doing in a bid to entice him to join them. The MC would be involved in the conversation, either arguing why it wouldn't work, or perhaps pretending to go along with them and offering up suggestions as a bid to gain time until he figures out how to get out of this/cavalry arrives/however it proceeds.
These are all good suggestions. I have a lot to think about.