My son, aged 12, is on the spectrum, along with several other related disorders, such as Sensory Integration Disorder. This year marked the 6th year in a row we've gone to Disney. Granted, everyone's experiences will be different and each child is unique.
As others have mentioned, the
DAS is going to be invaluable. We've done the varying forms of this over the past six years, from when it was a physical card to where it is now tied to your tickets. The ability to wait elsewhere rather than a queue is a lifesaver.
Limit your times in the parks. You don't necessarily need to spend a ton of time at the parks. We tend to spend 4-5 hours tops each day and take days off in-between park days. When he's had enough, he's had enough. Perhaps let the child help plan out their days and the attractions they want to see. Avoid overly frightening experiences such as Stitch's Great Escape and It's Tough to Be a Bug.
My son particularly has issues with social interaction. He will typically not interact with strangers at all, going so far as to step behind my back when he is addressed by others. However, the first year we went he decided that he wanted to meet characters and collect autographs. This has become one of the highlights of our trips each and every year. Despite not wanting to interact with any 'real humans', he will eagerly meet any and every character he can. I never say anything about his autism to the cast members of the characters, but they seem to know and let the interactions flow naturally. We've had some very magical character interactions over the years. Long-winded point here is--let the child dictate what they want to do and not do.
