Jeans
Menswear fashion blogs and sperm count studies have both proven that skinny jeans are not a good look for dudes over the age of 24. Your guy might not know that the trend clock for low-rise, peg-legged sausage cases ran out a few years ago, but fear not, easing him into a more flattering, straight-legged cut isn't going to be hard. For one, just have him avoid anything marketed as "skinny" or "super slim," unless he wants to dress like a tween. If he pushes back, just show him photos of Fall Out Boy. That always scares 'em straight.

For a first purchase, a pair of dark-blue, button-fly Levi's 501s is the gold standard. If he is a little fancier and on the trimmer side, have him spring for a pair of A.P.C. new standards, which are cut cleanly and sharply and tend to run a little French (if you know what I mean). No matter what he decides on, he will thank you when his cash, keys, and other prized possessions all have room to breathe. Plus, sexy time is much more fun and spontaneous when it doesn't require a pair of pliers to dispatch his offending nuthuggers. 

Levi's 501 Jeans, $48, available at Levi's; A.P.C. New Standard Jeans, $175, available at A.P.C..