Hey there, and

to our happy little home

! Sounds like you've already got a lot of good information, but I'll add what I can.
CERTAINLY use a stroller for both boys! Even a lot of NT kids use strollers around Disney at the age of 6. If you want to convince hubby, just tell him that the average Guest walks between 6-10 miles a day! Try doing that on legs that are less than two foot long

.
I know it would be more of a hassle, but you might want to consider taking your stroller for your younger son, and renting a single stroller for your older son. I think that letting each of the boys have their own space would releave a lot of stress. The double strollers don't have a divider between the two kids, so they would be sitting right next to each other. If your older son gets stressed (especially with the heat and humidity), it could lead to fights or meltdowns, which you really don't want to have to deal with.
Also, if your older son has his own space, it can provide a safe haven for him to retreat to when things get overwhelming. You can even get a stamp on the GAC that allows you to take the stroller into lines (stroller as wheelchair), so that he can have a physical barrier between him and the other people in the crowd. Some people find it helps to drape a towel over the stroller canopy when things get too much, to make a completely enclosed 'cacoon' away from everything. Just make sure he has a fan in there to keep cool! He doesn't have to use the stroller all the time; you can park it up in the sroller parking when he needs to burn off some energy.
The Fastpass system is really useful, and will help you miss some of the longest lines. September is a nice quiet time, and a lot of rides will be walk-on (i.e. no line at all

). For those that aren't, you can get a Fastpass ticket, go off and play elsewhere in the park, then come back between the allotted times and get on with very little wait!

You might want to go through this system with your son, before you leave, so he knows what to expect and won't meltdown when you start walking away from the ride without going on it! Oh, and you need a pass for each person who wants to ride, and each person can only hold one Fastpass at a time.
You guys will probably need to be even more careful than most young families about taking regular breaks. Any tired child can get cranky, and if you add that to pre-existing issues, you're not in for a smooth ride

. Go into the air-con at regular intervals, have lots to drink, and try to have your rests in quiet, low-stimulation places. Don't spend too long on Tom Sawyer's Island or at Animal Kingdom in one go, as neither of these place have very much air-conditioning, and the boys might over-heat. You may even decide to go back to the resort during the middle of the day, when it's hottest and busiest, to have a nap, a swim or just watch some cartoons.
Now on to the characters. Firstly, if he really can't handle them, you can just avoid taking him to them. They are often inside specially dedicated buildings, but when outside are off to the sides of walkways. For parades, they may come up to you, but if you give them a little head-shake, they should take the hint.
If you want to try meeting one, I would suggest picking a character from his favourite film, and try and find them in one of the inside spaces. Brief him before-hand, and make sure he knows that he can leave straight away if it gets too much for him. Explain to one of the handlers (the Cast Members that look after the characters) about your son's issues, and they can warn the character to take things slowly with him. It may be that he doesn't feel happy going up to them, but that's fine. Even if he just wants to stand and wave at them from a distance, nobody will mind. That said, I know that some people here have reported their Aspie kids running up to hug characters, when normally they would avoid contact with anyone where possible. Also, if it helps to set his mind at rest, you can tell him that I (supposedly a 'grown up'

) am still a bit scared by Mickey (long story).
Try to avoid doing the water parks or Magic Kingdom on a weekend, as this is when a lot of local residents will visit, so they tend to be busier. The water parks are best on slightly over-cast days, as they're cooler and quieter (just warn the boys that you have to get out
straight away if there is a thunderstorm warning broadcast over the speaker system).
Be prepared to do the same thing lots of times. I know a lot of Aspie kids love the train, for instance, and have to do three of four turns around the park before they're willing to do anything else. If necessary, you can split the group up sometimes, so that you can do different things; then meet up at a pre-arranged place and time. For instance, your younger one would probably enjoy the Playhouse Disney show, but it might be too stimulating for your older boy (lots of bright colours and music, and seating is just a large carpetted area).
Other than that, take things gently; be flexible with your plans; have some 'coping methods' planned out if things get too much (tell someone that you're feeling stressed, go into the stroller, cover your eyes, put ear-plugs in or listen to music on an MP3 player, get somewhere quiet, etc); make sure everyone has enough food, drink and sleep (a few emergency snacks and drinks might be a good idea); and most of all -
have fun!!
You'll be arriving the same day we do, so if you see a couple of mad Brits trundling around, with a blonde woman in a decorated-up blue wheelchair (that would be me) do say hi!
Wow that was a long post!
