There is no pass that will allow a party to bypass lines or not have to wait.
There is something called a Guest Assistance Card, which helps alert Cast Members to invisible disabilities. There is good information about the GAC in the FAQs sticky at the top of the forum.
When they go to get a GAC, if they choose to, they will not be asked for a diagnosis, but usually something phrased like this "What concerns do you have that would keep you from touring the park?"
Sometimes people cannot climb stairs, or have a visual impairment and need the front row of a theatre. There are many possibilities.
How old is your nephew? What needs does he have?
I can honestly say that as a person with ADHD myself, that a GAC is not a huge help. What helps me most is planning and distractions. I always have my Kindle with me so I can read in line, or I play games on my phone. Even with a GAC there will be a wait, and the alternative to the main queue is not always better for a person's needs.
If we know a little more about his family's concerns about touring the park, we may be able to give more specific advice.