Oh, Tina, December is my favorite time to visit Walt Disney World. It's just so beautiful. One of our traditions is just resort hopping on the monorail to look at all of the holiday decorations and do some shopping!
Because your son loved the monorail, and because of the gorgeous decorations that will be up, I would recommend
Disney's Contemporary Resort if I had to choose between the two. If you get a room in the main Tower section (as opposed to the Garden Wings which are not attached to the main building but still have the same room layouts), your son will be able to watch the monorails go by right from the hallway outside your room. Even as an adult, it's still pretty exciting to see!
The rooms in both resorts are almost identical, with different themes of
course. Depending on which side of the Tower you are on, you will have a view of the Magic Kingdom and the fireworks, or Bay Lake and the Electrical Water Pageant. I just stayed there in December with a Bay Lake view and it was truly magical. If you opt to stay in the Garden Wings at the Contemporary, you may
have garden, pool, marina, or parking lot views. With the Polynesian, you could get a view of Seven Seas Lagoon, the Magic Kingdom, or pools, gardens, or parking lots.
This is not to say the Polynesian isn't wonderful as well. If you are more into tropical decor and feel, definitely consider it. We really enjoy Kona Cafe and 'Ohana, but I just love the bustling feel at The Contemporary. I stayed at the Polynesian in February during a cold snap and it was pretty chilly to walk from the lobby to our room in the Tokelau building. Personally, I would just take the monorail over to the Polynesian and check out the decorations, shops and restaurants, but stay at the Contemporary.
I hope this helps you decide the resort for your family. Whichever you choose, have a wonderful vacation! ~Suzannah