Here is my advice, though my experience is my son on the severe side....
-be prepared to have a really good time. As I mentioned my son is on the severe side. The first time I took him I thought we *might* be able to stay for 20-30 minutes. I expected that we would have a look, ride a ride if we were lucky and leave. After one bump, we realized he LOVES WDW. We can go at opening and stay to close...and the next day he is ready to go again.
-But have contingency plans. For instance:
Print out the quiet area locations, and keep an eye out for out of the way places you can take a break if you need to.
Stop by First Aid and get a feel for it. We never visited first aid, until we needed a bandaid on the last trip. Turns our First Aid could have helped us out with a very quiet relaxed place when we needed it.
Keep your body comfortable to prevent overload. June is HOT. Have a plan to keep yourself cool and regulated.
I know this is kind of stereotyping, but I would have a plan in place for communication if you get overwhelmed or overheated. It's hard to plan out communication while in distress, and autism is on many levels a communication disorder. We know some older very high functioning folks with ASD who keep cards to hand out to a friend or responsible person when communicating maybe an issue, even as a way to order thoughts to communicate the distress.
My other piece of advice is watch crowd flows. We been at really busy times, including Christmas and even during huge crowds, people flow as a group. We watch the crowds....and flow elsewhere. We eat before lunch time and dinner to avoid crowds. We take a break when crowds start to peak. I like planning down to the minute, but even without planning, you can often avoid crowds by watching the flow.
Finally, if you need help ask. CM really do go out of their way to help find solutions.