Ahead or quill stem?
If it's Ahead, 1" stems aren't common; far easier to use a 1 1/8" stem with a shim to reduce it to 1".
If it's a quill stem, OS clamps aren't common (if available at all) and he'll need new handlebars that fit the stem, which will be either 26.4mm, 26.0mm or 25.4mm.
Alternatively, you can use a threaded-to-threadless converter as described above.
Do you already have a quill stem?