His sense of style may not be on the list of reasons you're marrying him, but don't worry — that's where you and the fashion rules come in.
His suit or tux should fit the formality.
At the most basic level, his attire should be appropriate for your venue and fit with the overall vibe of the wedding. If your wedding is in the daytime or outdoors, it can be a bit more casual (think: lighter-colored suits made of fabrics like seersucker or khaki). If it's an evening affair in a ballroom or swanky hotel, go with either a dark, well-tailored suit or for black-tie attire (a tux). Need to step it up one more notch? Suited for an extra-luxe venue, white tie is the ultimate in formalwear. This means a black tailcoat, white shirt and white bow tie.
His attire should coordinate with yours.
Also fairly obvious, but it has to be said: Your wedding is your first opportunity to show off your couple style, so make it a joint effort. While you may not actually want to try on the gown in front of him and compare styles before the wedding, you'll want to be sure your styles work together. So if your dress is a bedazzled ball gown, you won't want him wearing a lightweight linen suit; you'll want him in a classic black tux. Other combos that work: a rustic lace gown paired with a tailored tan suit, or a streamlined city-chic gown with a slim-cut gray suit.
His body type should dictate the suit.
The key to looking sharp is dressing for your body type. If he's tall and thin, most tuxedos and suits will look good on his frame. To add bulk, try a double-breasted suit, which will make him look broader. To slim down, try a fitted suit with a little bit of a nip in the waist to give the impression of a leaner silhouette.
His fit should be perfect.
Even the most expensive tux on the rack will look and feel awful if it doesn't fit right. He should be able to move around easily; have him do lots of twists, turns and arm raises to make sure there's plenty of mobility. Regardless of whether he's renting or buying, most shops will custom tailor the purchase.
Accessories should set him apart.
To achieve a unique look, it's all in the details. Spice things up and have your guy wear a special boutonniere or a bow tie, vest, tie, cummerbund or cuff links in a different color or style. If your wedding palette has two colors, he can wear one of the shades while the rest of the groomsmen wear the other.
Encourage him to inject his personality and show off that special something you fell in love with. If he's musical, add a treble clef to his boutonniere; if he's a lovable sci-fi nerd, gift him with Star Wars cuff links.