My grandson likes his wooden train set. Blocks. Books. Cars and trucks. He doesn't mind having a few stuffed animals and gnomes around, but he relates to them differently than his big sister. Today I suggested that he read a story to one of his dolls. "Dolls are just stuffed toys." End of discussion. On the other hand, he is really good at pretending to cut the grass with his toy lawnmower.
He also likes puzzles, his marble game (slanted things that marbles run down and down and down) and absolutely anything mechanical. He is almost 4.
As to decor, he shares a room with his sister, so he is used to frou frou, because she is about as frou frou as they come. He currently sleeps under a quilt I made for her, which is entirely made of pink prints. He hasn't caught on yet that pink is for girls
So, for decor, why not just use colors that he likes, have him paint some pictures of stuff he finds interesting to hang on the wall--pirates are fine if he paints them himself rather than buying a media linked poster--and perhaps get a bed covering and curtains that have a theme he likes. You might have to shop around a bit to find something that isn't "licensed" and linked to a movie, TV show, or other product. I hate the system that makes us pay to advertise their junk!