the above is excellent advice. There is a lot of info out there for people with kids on the spectrum visiting WDW, there are even a few facebook pages.
I would suggest trying one of the more low key character meals first, and then if that goes well do others or cancel them. We like Garden Grill, the food is brought to you which is easier for those with children, and the restaurant isn't as loud as others.
Be sure to get a DAS for your relative. Be prepared to explain why he can't wait in long lines. Just saying he's autustic won't be enough of a reason. They also have a stroller as wheelchair pass that we use, not sure how old the child is, this has been very helpful to give our daughter her own space. This allows us to take the stroller on lines and in to shows.
Have a wonderful trip. Feel free to reach out with any specific questions!