Actually, the week right before Christmas is a time when less expensive hotel rooms are easily acquired on sites such as quikbook.com, travelzoo, expedia, etc.

Guys, you have been really helpful, thank you! Also 'christnp' thanks for the heads up on the lack of elevators on the subway.
Cheers!

If lack of elevators on the subway are a concern, all city buses have lifts to enable wheelchair access. I believe they can be activated for any person unable to climb the stairs, whether or not they are in a wheelchair.

A few subway stations have escalators or elevators - but they are often not working. And since there are multiple subway lines, sometime crisscrossing under each other, getting from the street down to the actual platform can be 3 double flights of stairs.

Buses can kneel for wheelchairs - don;t know if they will for others. However, often all seats are taken (typically run 100 to 150% full except at odd hours, and around the holidays bus travel is VERY slow compared to subway (walking is often faster than buses).

I don't think anyone's mentioned yet how crowded the sidewalks can be during the Christmas period. The week before Christmas is my favourite time in NY -- it's beautiful and exciting, but it is quite a crush. If your son tires easily or is in a wheelchair (you don't say), the sheer volume of people may be a lot for you to deal with.

That said, is it possible for you to do this whole visit through a tour company -- one who will arrange hotels, events and city tours, etc., for a better 'group rate'? I've never done this myself but there must be reputable firms who do it. Perhaps the Fodorites can suggest a few.