It depends. What other works has he read so far? Is he a rebellious type of kid? What do his friends read? What sort of yeshiva does he attend? Is he ‘conventional’ in terms of his social standing with the other bochrim?
And what, if anything, gives you pause in terms of Tolkien?
For example, the Little House books by Wilder are generally considered wholesome (apart from the contemporary controversy over her treatment of Native Americans, which is an issue in itself) but many frum parents might be uncomfortable with their children reading about the characters’ Xmas celebrations. The Arthurian Legends are considered classic reading in the outside world, but I personally wouldn’t want my children reading them any more than I would want them reading Greek mythology, as religious matter of course.
You have to trust yourself.