There is a blessing and curse with first visits - the blessing is that because the person has never been, their expectations are somewhat open... except... there are few people who *haven't* seen those beautiful Disney World commercials on TV where it looks like walkways are a mile wide, and no one else is there, but you and your family! And there are no lines anywhere! And everyone is smiling and happy, and no one is having a meltdown, and no one is sweaty and grumpy and so tired they would kill a Muppet for that bench, give me the bench NOW, and you better make it shady.
And that's the curse. Unrealistic expectations set by cinematic television commercials and those "Vacation Planning" DVDs that Disney used to (do they still?) send out. (I know that you can download the whole thing from the website and watch it on your devices now)
Possibly one of the greatest "gifts" you can give your nephew will be to just set expectations. Let him know that there are going to be a LOT of people there (especially in the Magic Kingdom) and that he might *feel* the fireworks when they are set off. (Our daughter is actually quite sensitive to those sensations; we had to move way back down Main Street about half way through HEA in January). Talk to him in advance about holding hands (tell him Mickey & Minnie hold hands all the time

) with someone in the family ALL the time. Work with his family about things like Resort rooms (WDW has rooms with hearing-impaired features, like flashing lights for smoke detectors, etc.)
And then have the best, first-ever trip