they also said that the GAC trumps any Fastpass tickets..............................
I wanted to clarify this statement. If you have a GAC that allows use of the fastpass line, you can wait in that line, but you don't get to go ahead of people already waiting in the fastpass line.
And (this is more for other readers than the OP), not all GACS say the same thing.
For situations where people can't wait in line without a lot of stress (such as someone with autism), there is a stamped message on the GAC that allows use of the fastpass return line. That stamped message is not used for everyone who gets a GAC, just those who really need it. The standard message printed on the GAC says that the user is expected to get and use fastpasses as much as possible if they want to shorten their wait in line. Then there are stamped messages that give different instructions based on the needs identified. Some people need only to have a waiting place out of the sun, to bring a stroller in lines or need to avoid stairs - their GAC would have a message that addresses those problems.
Also, the GAC card does say that it is not meant to give immediate access to rides - sometimes (even with a GAC that says you may use the fastpass line), you will be issued a "ticket" that allows you to come back into the fastpass line at a later time. In that situation, they usually make the return time the same as the posted time for the standby line.
We use a GAC for DD who has multiple disabilities. On some trips (even during Easter time, which will be similar to the last week of December when you are going) we only need it one or two times per day. Even though some of the really popular, low capacity rides have long lines, many of the others don't. If the standby wait time is 15 minutes or less, we don't use the GAC at all (the wait in the fastpass line is going to be 15 minutes or less).
And because many people are so tuned into getting fastpasses, they don't look at the time on the standby line - they just assume it is long. At Easter, my DH, other DD and friend wanted to ride TOT. The fastpass return time was about 2 hours in the future if they had gotten fastpasses. The standby line wait was less than 15 minutes. The same thing happened at ITTBAB, Indiana Jones Stunt Show and several other shows.
If you are able to get to the parks early in the day, you can ride many things without a wait at all. DH and I went on an adult only trip in mid-July (when it was really crowded). We got to MK by a little after opening and were able to go on all the fantasyland rides (except Dumbo and Tea cups) plus HM by 10:30 am without using fastpass or doing any commando touring.
2 other hints - bring earplugs and consider a stroller. Some of the rides have loud soundtracks and if you DS is sensitive to sound, he will appreciate those. Also, a lot of people like the WDW strollers for children with senstitivity issues. They come in sizes big enough to fit up to a small 12 year old, are molded plastic and provide kind of a safe haven cocoon.
Also, if you have the option to change when you are going, you might want to consider going earlier in December or further in January. The last week of December up thru New Years is a very busy time in the parks and just the busyness of the place will provide a lot of extra stimulation, especially since this is your first trip and everything will be unfamiliar (think of Christmas shopping in a busy mall).
