The party is sooooo much fun, you guys will have a great time! There should be parties running by the end of September, so you'll be fine. We went to one on the 15th last year, and I don't think that was the first one of the year, either. The demand for the earlier ones isn't as high as for those near Halloween itself, so you shouldn't have any problems getting tickets (I think we only got ours a week or two before we flew out).
We went for a party in the middle of the week, and the crowds weren't too bad at all. We didn't do all the character meet and greets, but we found that lines for those and the Trick or Treat stations all moved pretty quickly. If everyone in your party can cope with a bit of a late night, the second parade is a lot less crowded than the first. Whichever one you go to, I'd suggest you use your GAC to sit in one of the wheelchair viewing areas, so you can keep your son in the stroller.
You might want to plan a very gentle time during the day itself. I've got fatigue issues, so we did this, and I was very glad we did. We just had a late morning, did our laundry and had a nice long swim in the pool. We stayed right through until about midnight, and the crowds were a lot easier after the fireworks had finished.
As for costumes (you've all got to do costumes, it just makes it so much more fun!), make sure you've not got anything too hot or constrictive (even more important with a kid with ASD). Also, no long tassels that can get caught in travelators or rides

! I wouldn't reccomend masks either; I had a Mardi Gras mask, but I put it on a stick so it wasn't strapped to my face.
If you get given more candy than you want to eat / take home, the Disney hotels will take it. We handed ours in at the Guest Relations desk and they were very grateful for it, as they hand candy out to young resort Guests on party nights.
Have a great time!
