Personally, I would not take a 5 year old to a show that runs upwards of two and a half hours. However, if the child is well-behaved and can sit quietly through the entire show, there shouldn't be an issue. I don't know this particular child, but most 5 year olds that I have met could probably not do that.
Also keep in mind that the parts where The Wizard enters and when the monkeys fly around could be potentially scary to such a young child.
Essentially, if his behavior could be disruptive, do not take him.