As long as you don't exceed the lawful occupancy of the room, there will be no extra charge to have your minor son added to your reservation. The only charge will be for park tickets, but you'll be paying for those one way or another anyway.
Did you sign up for the Dining Plan? If you did, and you're trying to avoid having to pay for his dining plan, I'll echo the facts the others presented to you.
He won't be able to be named on the reservation. Which means if you are going to take Disney's Magical Express to and from the airport, he'll have to find alternate transportation separately from the family, and pay for it. He won't have a room key. He won't be able to have room charging privileges (if you wanted him to have that, anyway). He won't be able to participate in Extra Magic Hours at the theme parks and water parks.