My daughter has high functioning autism (level 1 with language impairment). Like most people with autism, she has an obsession that she fixates on and for us that just happens to be Disney movies and characters. That said, its impossible for me to tell you how difficult or not it might be for your nephew. We also bought the dining plan and primarily did character meals (Akershus, Chef Mickeys, Crystal Palace, and Hollywood and Dine). My daughter was so excited by the characters that it helped her get through a busy, overwhelming meal that if the characters hadn't of been there I don't think she would have been able to do. I was amazed at how well she did. It's going to depend on his personality.
For shows and parades we needed darkening sunglasses and heavy duty ear protection, and needed to stay towards the back and as far away from the action as possible, but with that she did fine as long as she was in her own personal space bubble. Most people with autism truly need a defined personal space (which is why the single stroller is so helpful), so as long as she was in her own theater seat or stroller she did okay.
There will be rides that he won't want to do and my advice would be not to push anything. Let him decide what he wants to do and just go with that. My little one had a huge melt down and panic attack and refused (kicking, screaming, rolling on the ground) to go on Little Mermaid, Pooh, or Nemo rides, even though I know she would have loved it. We also rode "Donald's Boat Ride" (the real name of this is alluding me after having listening to her call it that so many times) approximately 30 times during the vacation. Once we learned that we just had to say "okay, no problem" any time she said no to something, our vacation turned super magical.
When it comes to character interactions, you'll also need to follow his lead. My theory after talking to other parents and watching my daughter is that there may be a big difference between face and costumed characters depending upon his level of comfort with verbal and non-verbal language. My daughter struggles very much with comprehending and using spoken language and character interaction with the princesses was very awkward and uncomfortable. On the other hand she LOVED the costumed characters. The fact that they used only body language to communicate really worked with her. At the same time, many people with autism are reversed from my kiddo - they struggle with body language and do better with spoken.
I hope you guys have a great trip! My best advice for traveling with an autistic kiddo is to prepare for all of their needs ahead of time; locate quiet locations, bring favorite foods, bring the comfort items and headphones. After that, let them just set the pace.