I would do exactly as he asks. Does anyone need to know that you know? If not, after his death, just lie and say that the death was as big a surprise to you as to others.
If they know you would know, just say quite simply that when someone shares information with you and tells you explicitly to tell no-one, then you do as they asked, you do not betray your friends/family.
After someone has died my reaction is the exact opposite. Funeral plans are to comfort the living. Knowing you will be buried with a spouse, or in a particular way are a comfort to the living about how they will be treated in death. Once dead they have no means of knowing how their body will be disposed of or doing anything about it. so family and friends should feel free to do things as they will and ignore the pre-death wishes.
I have told DC exactly how I wish to be disposed of after death, what ceremonies are to be performed etc, but I have also given them freedom to ignore them entirely, without guilt, should fulfilling them cause too many problems.
Having said that, I would always respect and follow the post death wishes of anyone whose disposal was my responsibility, but if for any reason they could not be followed I would not waste anytime feeling guilty or upset about this.