Often, a man's home office is his castle. It's the only space in which he gets to put his particular imprint. For that reason, you can't go overboard on steeping the room in as much testosterone as possible—it should ooze from the walls. Achieving the look doesn't have to mean lots of dark brown and rust, but it does mean keeping out knickknacks, plants (they'll never make it), pots of potpourri, and lilac-scented candles.
You should include anything that evokes his most masculine hobbies. Does he like single malt scotch? Put in a nice bar with shot glasses and heavy crystal decanters. Is he a golfer? Display his prized golf balls signed by the greats in a wooden frame with slots made to order, and have it lined with green felt.
There's a lot of room for you to get creative when you start decorating the male home office. Even though this might be the one room in which he'll want to have some say in the décor, it can be fun to take a space and make it masculine. It's a way for you to really get inside of your man's head. On the other hand, since it's going to be his space, it's also a good opportunity for the two of you to communicate. You'll have to give him the stage and really listen to his ideas about how he wants his office to look. When he's finished giving you his vision, tell him your own—he may be surprised at how good your ideas sound.
It's important to remember that dark is no longer the only way to create the masculine look. Earth tones for paint color, and hard materials, for instance, steel, brick, and rock, can make a place take on a distinct masculine cast. Leather has always been a popular masculine textile for interior decoration and still works well, in part because it's sturdy and looks good forever and also because it's easy to clean.
Clean and sleek is another way to go in getting your home office to scream, "Man." You can get that look by using cool shades of gray on three walls, with a fourth wall faced with stone—try a dark slate for masculine contrast. With that done, you can use wide planks for flooring in ebony or jet black. You can top it off with a gray area rug to provide more texture and contrast. Get some shutters for the windows instead of a fussy window treatment. Keep the theme in a woody ebony shade. All that's left is the lighting. Here you can add in some steel lamps you can angle so your man can have lighting wherever and whenever he chooses.