Get a friendship bracelet kit for funsies. These are the bracelets where each bead is a letter, a heart or a cheap plastic thing. Tell him “I’m making friendship bracelets to put in the stockings for Christmas. I need to practice on you” Make him one. Tell him not to shower with it because the metal is bad quality. You now have his wrist size.

You can also memorize the bracelet order and aragement of the beads and make a copy when he’s gone.