The part of confidant can be a hard road. Your brother in-law knows his family, he knows his health, and has likely spoken to the doctors about options. Most of the time a kidney donor is a family relation, (at least that is my understanding), and he knows his family and likely their situations. He needed someone to talk to outside the family but a part of the family, he most likely does not want to be a burden, I can only guess, I am not there nor a family member.

There could be many reason for him not getting a transplant, personal choice and medical choice, are but two. The medical reason could be survival odds, or other underlying medical conditions that make him not a good candidate for a transplant.

Right now it seems he likely just needs someone to talk to, to carry his words and wishes to the family when the time is right. It is a hard road, but it is a road he has chosen, and one he has asked you to follow along on. Let him know if it becomes to much, talk to him and help him if he wants it, but also be careful about becoming a crutch for him. Let him know when/if it becomes to hard to keep his secret and that you would like to be able to talk to so and so about it, if it ever gets to that point for you. He will understand, and likely accept you need to bring someone else in on his decision.