If you add him now as a guest on a package reservation that includes tickets and DDP, he must purchase the dining plan for the length of stay AND he must purchase the same type and length of theme park ticket as everyone else on the reservation.
You could buy a one-day ticket for all of you and then upgrade yours. You'd have a one-day ticket left over if your dad does not intend to use any park tickets, but they never expire. It doesn't matter what type or length of ticket you buy, but everyone in the room must buy tickets of the same type and length.
If you get the dining plan, and he is on the reservation, you'll have to get the dining plan for him, too - for the full length of the stay. Yes, after he leaves you can use the credits.
I do not know if you will be permitted to add him to the reservation when you check in. (If you plan to use the Magical Express airport transportation service, he will not be able to use it unless he is registered on the reservation before you arrive.)