First I would explain to the peasant boy that he does not die for the contents of the letter, but for the adherence to the protocol of delivery. It would therefore be devoid of any sense to open the letter now and thus thwart the delivery of the letter according to protocol forever. Then he would indeed have died in vain. I would promise him that I would finish delivering the letter in accordance with the protocol, so that his journey before the injury would not have been in vain, but part of a whole. Namely, the delivery of the letter according to protocol. If the peasant boy is too stubborn for this realisation, he deserves to die as he has lived, ignorant of the truth. Thus I would deny him any knowledge of the contents of the letter.
If I thought the peasant boy was clever, I would start explaining to him that life is not about results, but about how hard you try. So as long as the lad tried hard, he need not worry about the result.