Paint the walls a sea blue. Put up those removable stickers with treasure chests, fish and items from the sea. Get a whale chair and a toy box that looks like a treasure chest. Get pirate themed bed spread and curtains. Get an anchor to hang on the wall. Fun!
To find pirate themed furniture all it takes is a quick internet search or a couple of phone calls to your local furniture store. You could also find a great wallpaper with a pirate themed design on it that Im sure your son will enjoy. There are tons of ways to find great pieces to add yo his bedroom.