Hi Christine!
You have a few options for accommodations and it will depend on your family's interests and budget for your stay. Since you have a family of 5 and require space for your 7 year old son, I would suggest you skip the option of a room at Port Orleans Riverside, since the trundle bed room won't give you much more space than a standard room.
To start, you have the option of a Family Suite at All Star Music Resort, which has the feel of two rooms (and a kitchenette that I find useful when traveling with kids). Aside from the queen size bed in the "main" room, there are sleeper sofas and sleeper chairs. This is a good option especially if you're on a budget.
Also, there are cabins at Ft. Wilderness Resort. They have a "main bedroom" with a double bed and bunk beds, and then a "murphy" bed in the living room area, along with a full kitchen and even a patio area. The cabins offer you a nice amount of space, and when I'm there it feels even more roomy because of the outdoors.
If you would to like stay at a deluxe villa, then you have a whole set of options available. There are villas that offer studios all the way through to 3 bedroom accommodations. You can see more of them
here. Which one you select from the villas depends more on which resort you like best and where you'd like to be located.
Depending on which kind of resort you think will be the best fit for your family, you can probably start narrowing down your choices. Hoping this helps your family, and that each of you has a great time on your magical vacation!