Thanks for everyone's comments. I've enjoyed reading them. In answer to some of the questions:
The narrator has been in the camp long enough to have heard the stories about escape attempts, and what one needs for them. So he understands Dmitri's plans for the boy from the beginning. This is the main factor in his initial reluctance to participate. But I think that he allows himself to indulge in some magical thinking--that they will be able to walk straight through the forest, that the exiles won't be as bad as the stories say, that he will be able to come up with some alternate solution, etc. etc. His desire to get out of the camp is so strong that deceives himself about the consequences of his decision--oh, it can't possibly get that bad. I know I've done this myself, though the result has never been quite so disastrous.
And yes, Dmitri has taken the narrator's measure, and is certain of the choice he will make. He forces the decision on the narrator to ensure his complicity in the crime, to demonstrate his point about the true nature of the narrator's idealism, and, as eytanz perceptively points out, to create a pathological bond between them that he hopes to exploit in the future.
I am finishing up my first novel right now, and I was considering, for my second, to expand this story to novel length. This would be the first chapter. The remainder of the book would involve the narrator's attempts, on his return to civilization and the revolution, to redeem himself somehow--to try both to absolve himself of his sins and to escape Dmitri's grasp.
Anyway, thanks again to everyone for listening and commenting. My short time here at Pseudopod has been a lot of fun.
John