If there is any question as to whether or not someone might be hurt, how can this even be a legitimate question as to whether or not he should participate. As noted above, more than a guy dressed up as Vader, there are 11 young kids on stage. I personally wouldn't chance that my son would ruin it for everyone else, or god forbid, hurt another child because I wanted him to experience something that I knew wasn't right for him. The fact that the question is being asked, to me, means that you know what the right thing to do is for your nephew. Follow your gut, because you know he shouldn't participate.
Not all experiences are right for everyone, and I think we have a duty to others to know our kids (or nephew's) limits and act accordingly.
Not all experiences are right for everyone, and I think we have a duty to others to know our kids (or nephew's) limits and act accordingly.