Disclaimer: I am in no way affiliated with any of these types of products.
There is at least one book, "I'm dead, now what?" which contains questions about end of life decisions and spaces to answer. Some of the questions provide some comic relief, making this somewhat awkward conversation a little bit easier to deal with. The books are quite thorough and cover topics like life support, who gets what, preexisting burial plans, and more.
Now that they are starting this conversation with you, I would either get a pair of books like this for your parents as a gift, or suggest to them that they look at them. The humor included in some of the questions may be enough to persuade him to fill it out. Some people I know love making lists, the fact that this type of book lists out the questions for them has encouraged them to fill it out.
My personal experience with the book doesn't pertain to this situation very well. I just thought it was a good thing to have around, just in case.
If the humor and lists do not persuade him to help, you may benefit from picking a copy up for yourself. It may help you by showing important questions that you have not thought of. You could also keep these questions in mind while spending time with your parents - they may answer them over the course of normal conversation (this will take time). If all else fails having a copy for yourself will allow you to slowly insert the important questions into your conversations, avoiding the need to know everything right now.
If they do decide to fill them out, you do not need to be present, but you should know where to find them in the event that the books are needed.