This is not over cost perspectives, but rather, the themeing.....and I believe at your sons age, the larger than life icons of some Disney film legends alone would entice your son to want to even stay put right there....my kids who were older when we went last time were totally enthralled over Buzz,Pongo and Perdita...and even more so, still talk about the giant Mickey Socerer hat...they really saw this stuff as being sooooooo cool, and when they found out that you could walk through the hat...well, lets just say, it was a focal point anytime we were near it.
That being said, if I were to choose a moderate, I would have a hard time choosing...each has its own merits..but personally, (and maybe because I spent a long time deliberating this for our trip this time) I went with Coranado Springs...most of the reason for that is the pool/dig site area....if you havent seen anything about this resort, there are a few people here who have entire sites devoted to it..pics alone sold me.
If I were to stay on the monorail.....I would have to choose the Contemporary, based solely on location, but I would have to have a veiw of the castle....(of course, it would have to also be a moneys no object vacation!!)
Finally, if I couldnt decide, I would give into temptation to stay at a couple of different locations...maybe split the trip between staying at the least expensive, then go for either moderate or deluxe.
My final words and advise on this...you know yourself what your son loves...if he's into Disney movies, well, the choice is obvious....if he loves animals, again, pretty easy.
I would show HIM what he can choose, and let HIM decide...call me crazy, but this choice may be left better to the professional...your son!!!!!!!
Whatever you decide, have a great time bonding with the little guy...it should be a great time(and dont forget, if it rains, play in it!!!!Nothing better than jumping in a few puddles at that age, and he'll love you for it as well!!!)
A dad