You know how when your guy tells you you're sexy, it makes you want to dress sexy? When it comes to style, confidence can make all the difference, and what goes for you goes for him.
Tell him he has great arms ---and wouldn't they look amazing in that slim-fit Armani dress shirt? And it's a shame to hide his great abs in that ratty old T-shirt, drown his great butt in those baggy jeans or, perhaps, fail to show off his amazing hazel eyes by passing on that green-blue tie that's remarkably like the one Brad Pitt wore down the red carpet and which he would look even better in.
Tell him how great he looks in the pieces you pick out, and he might start to like his new style.