My friend is Dying of Cancer and wants me to oversee his Medical and Personal Obligations,?

We made the Decision together that a Hospice is the best place for him, He wants me to take over the Decision Making in all his Personal Affairs, He has no family at all, but he has a God Son but cant remember his name or where he was from, How do I go about Writing a letter to Inform People of his Decision? which will be legal and withstanding, He's been a good friend for 11 years and lives in an old caravan, I'd be honoured to take this on, but don't know how? or what to put in the letter for him to sign, Can anyone help?