Men typically do not take much of role when it comes to designing their living spaces. Often the woman takes over, this is her area of expertise. Women, you can be empowered to design a space for your man even if he claims he wants no part of it.
To begin, how would you describe a masculine space? It contains rich textures and deep colors. It combines clean lines and functionality. Sometimes quirkiness with the addition of personal items or objects (super hero figurines).
How do you design a masculine space? Combine the old with the new. Old beat up leather suitcases stacked next to a new, crisp fabric upholstered sofa. Think of iconic masculine environments, did they consist of neutral colors on the walls? Chocolates, taupes and grays are good bets. . A mix of metals, wood and fabric is fail safe. Matching all of the wood in the same stain is boring. Men's suiting fabric is always masculine. Need to soften the space, add plants. The opportunity to interact with the space, watering and taking care of a plant, gives a man something to take pride in.
Allow the man to express his ideas and take it from there. His office is a good place to start.