That's when he finds an ancient book of wizardry, & ends up summoning a "demon" - an alien from a parallel universe. Demon will grant his wish, or rather far more than he wished...
Fact sheet.First published: If, June 1954.
Download full text from Project Gutenberg, Manybooks, Feedbooks. [via QuasarDragon]
Download audio from LibriVox.
Related: Stories of Lester Del Rey.