L.A.
First of all really listen to any of the things he is speaking about, Let him know he can tell you anything.
There are many great books about death and about the cycle of life. I noticed our daughter always did well when we read a book on the subject. I would just read it and then ask her, what do you think.. Sometimes it would be another day when she might bring up a thought or a feeling.
His life is unique, but not unusual. He needs to know he is not alone and not the only person going through this.
Let him know you love him and are proud of him for being brave through all of this, but if he is worried or a little freaked out, that is to be expected. Just remind him to always talk about what he is feeling or worried about. You are not there to fix things for him, but to support him.