I don't know that there is anything that can be done, honestly. If the hotel is full, especially, it takes a bit AFTER a guest checks out (11am) for the room to be cleaned, etc.
What time do you arrive? I would suggest calling the hotel a few days prior (or faxing a note). Explain the situation, and hope for the best.
Have a backup plan, in case your room is not ready early. I'd not plan for the parks, but there is a lot you can do to occupy a young one for a few hours, if necessary, and much of it can be very calming/relaxing.
I see you'll be at the Poly. VERY convenient! Your son probably loves trains, right? (don't all ASD kids?). He will probably be fascinated by the monorail! You can ride it around and around as long as you want. OR, you can get off and "explore" the different hotels. Or, if that's not something your son would like, just take a walk to the Poly beach and have a seat in one of the hammocks or swing benches. Some gentle swinging might be calming, and you can see the castle from there, and watch the monorails go around. I think it would be really a nice way to pass the time.
I know what you're saying, though. I have a 3.5 year old autistic son, and we did a 3 night stay at DLR last October (his first hotel stay) and since we live so close, we purposely did not leave for the hotel until 2:30pm (check in time is 3pm at GCH), so when we arrived, we knew our room would be ready right away.