As long as it will not exceed the maximum number of people for the room, and you are at least 3 days out or so, then you can do that with no problem. He will have to have the same tickets as everyone else in the package. And, as others have stated, if he is a 3rd adult, you will have an "extra adult" fee per night, as well. Even though you are paid in full, you will not be charged a change fee since you are upgrading your package, so to speak. Good luck!