Hi Debra! What a magical vacation you are planning for your son! The Grand Floridian is an excellent resort choice since you'll be using a stroller for your son -- you'll be able to roll the stroller on and off the monorail and straight to your room when you're traveling to and from the Magic Kingdom and Epcot. I think you'll find that the Grand Floridian is more compact than most of the other resorts at Disney World, and you won't have terribly long walks to any of the rooms.
The Grand Floridian has rooms with three different views: garden, lagoon, and theme park. In order to have a view of the Magic Kingdom and see the fireworks over Cinderella's Castle, you generally must book a theme park view. Lagoon-view rooms may face other areas such as the Polynesian or the marina. If seeing the fireworks from your room is very important to you, make sure that you have booked a theme-park view and specifically request a view of the Castle. (If that room type is out of your price range, it wouldn't hurt to explain your son's situation and request a room with a fireworks view in hopes that the room assigner sprinkles a little pixie dust your way.) While the Sago Key building appears closest to the Main Building on a map, when you look at the distance you'll actually walk from the monorail station to a lagoon view room, I think it's about the same as the other buildings. The lagoon view rooms at Conch Key may be the farthest walk from the monorail, but arguably have the best views of the Magic Kingdom. The book, "The Unofficial Guide to Walt Disney World," recommends the following Conch Key rooms for views of the Magic Kingdom: 7228-7231, 7328-7331 and 7425-7431. In Sago Key, the rooms on the end of the building have a view of the Castle: 5139, 5144, 5145, 5242-5245 and 5242-5345.
Hope this helps! Have a wonderful time!