Dark Lord of Derkholm, by Diana Wynne Jones. (The link is to the Wikipedia entry, which informs me that the British editions of this book begin the title with the word "the." I don't know why anyone thought omitting that word would make any difference in the sales figures on the American site of the Atlantic.)
The basic premise is much as you describe. Allow me to quote two quick paragraphs from the Wikipedia entry to illustrate that we definitely are talking about the same novel.
A fantasy world is dominated by its destructive tourist industry. "Mr.
Chesney's Pilgrim Parties" arrange for annual group tours, evidently
from our world, to experience all the cliches: wise Wizard Guides,
attacks from Leathery-Winged Avians, the Glamorous Enchantress, the
evil Dark Lord. It is a devastating show: farmlands are laid waste,
people slain, and so on.
The head of Wizards University, Querida, determines a way to end the
tours. The apparently incompetent wizard Derk will be the next Dark
Lord, and son Blade the Wizard Guide for the final tour. Querida
overcomes objections all around and the plan is underway.
Blade is the name of the adolescent boy you remembered -- the kid who is assigned to be a guide for some of those tourists from our world. (As I recall, he uses magic to grow a beard so he will look old enough to be taken seriously.)
The whole story is something of a spoof of the popular idea that a small group of individuals can be banded together on the spur of the moment and somehow successfully complete the Great Quest to destroy the Dark Lord, thereby supposedly freeing the rest of this fantasy world from his evil influence. (You may have heard that J.R.R. Tolkien once wrote an epic fantasy with a plot that proceeded along those general lines . . . and his approach has been imitated at least once or twice since then. Perhaps more often than that! Apparently, Diana Wynne Jones felt it was time to shake things up a bit by suggesting such things are likely to be staged.)