My son is 10 years old.
We drive and he has no issues with the drive at all. (An occasional motion sickness, but this is rare.) We usually take our portable DVD player, but although we had it with us this time, he didn't use it at all. Frequent breaks on the drive, about every two hours, to get out and run around a bit helped all of us.
Once at Disney, we took the parks at a leisurely pace. My son does not walk fast unless he wants to and we did not use a stroller or wheelchair for him. We didn't try to rush anything and if we noticed him getting flushed from the heat, we took a break inside air conditioning. My son will not normally drink water, but while in the parks, he would drink it regularly without complaint, so I think the body kicks in and accepts things out of the norm if its a physical need. (I hope that made sense.)
He did have some issues with the rides as far as noise and darkness. We brought a very small light-up fan that he wore around his neck to give him a sense of security if the ride was too dark. The funny part is that we thought he would concentrate on the lights instead of the darkness, but instead, he held it out in front of him and used it to ward off the darkness.
We did get a guest assistance card to use so that we had quick access to exits in the event he got overwhelmed by an attraction or ride and we needed to make a hasty exit. He has a tendency to bolt if he's uncomfortable, but we never knew what would actually make him bolt. We did a hasty exit from the Muppets as soon as it started, and we watched the Philharmonic and Voyage of the Little Mermaid from a side corner near the exit. The intensity of the 3-D was not something he enjoyed.
In past years, he would scream like a banshee when he was afraid and I was horrified that it would be a huge bother to those around us on the attractions. This year, he didn't scream, but instead held his little light-up fan in front of him while telling us "Not fair! NOT FAIR!" I'm happy to report that those around us each time he had his bolts and/or meltdowns only looked at us with kindness and I did not notice any annoyed looks.
I really think your nephew will have a wonderful time!