Full custody and power of attorney are two different things, though full custody would give you 'power of attorney', though it wouldn't be called that. It would be legal guardianship (among other things).
Is it really full custody, or does he just want you to have power of attorney? PoA can still be limiting in a lot of situations with this.
You'll probably want to go with legal guardianship.
Look for "Declaration of Guardian In The Event of Incompetence or Need of Guardian".
You're going to want to have a lawyer handle this...