Don't: Scare the Crap Out of Him
If there's one thing that'll nip a style makeover in the bud, it's trying to replace a guy's entire wardrobe overnight. You are not Tim Gunn, and your sweatpant-wearing guy is not going to turn into a sharp-dressed man tomorrow.
An all-at-once approach will most likely result in discomfort and balking. Nobody's aiming for a man who looks great and feels awful.
Instead, view the transformation as a long-term work in progress.