This is great info!
It sounds like you will have 3 park days? My initial thought is, consider 2 MK days and 1 AK day. If your DS9 really loves the castle intro, doing MK two mornings will give you the chance to see it twice. Have you considered a meal in Cinderella Castle? If your son loves the castle, this might be really fun for him. It is a princess character meal, but the princesses really dote on the boys. My DS9 has been to all of them and walked out of one with a big Ariel lipstick stain on his cheek - he loved it and it was hysterical! If at all interested, you can even take him to BBB for a prince makeover, which DS enjoyed as well. It's right inside the castle and they spike the boys hair, put glitter in it and give them swords. We did that followed by a CRT lunch one day.
For the fireworks, if you can swing it, I would highly recommend one of the MK fireworks dessert parties (we prefer the one with Plaza Garden viewing). With the dessert party, you start with a dessert buffet at Tomorrowland Terrace and, if you choose the garden viewing, about 15 minutes before the fireworks, they escort the party guests to a private garden area without crowds and with a beautiful view of the fireworks. This is the only way we will see the fireworks at this point. My DS9 also has special needs and has difficulty waiting in the crowds for long periods of time. He also has difficulty with the crowds after the fireworks, most of whom are heading for the exit at the same time. It's very congested and it makes him incredibly anxious. With the party, you can see the fireworks without waiting, without a crowd and you can sit in the garden area afterward and wait for the crowd to disperse.
I would consider getting a
DAS pass for your son. It's the Disability Access Service pass and will help with lines a little if you feel you need it. You ask for the pass at Guest Services. If you need details about the pass I would recommend looking on the DISabilities forum on this site or I'm happy to get into more detail with you on this thread. We use the pass in conjunction with FP+ and it works well for us.
Will you go back to your condo during the day to rest? If so, I would seriously consider staying onsite just to make your commute shorter. My DS9 can only handle the parks for about 1/2 the day and then we need to decompress in the afternoon and head out for another park or event at night. We typically do late morning through about 2 pm and then head back to the resort. I would definitely recommend thinking about what your own families touring stamina may be and plan your days accordingly.