There are enough activities for kids on Disney, that your son would not be bored, and the shows are amazing! However, Disney is pricey, and there's a lot of you going. A good compromise would be Royal Carribean. We went on the Allure of the Seas, but Oasis of the Seas is the sister ship and it would be good too. They would have something for everyone.
We now prefer RCL over Carnival when traveling with the kids as we found the kids club to be better. If you do go with Carnival, make sure you pick a newer ship - perhaps the Dream or the Magic.