New curtains are always a quick way to update the look of a room.
You could applique or embroider or stencil anchors & boats etc on some of his linen
Also those transfers that go around the top of the wall.
Maybe a big plush parrot to make the room look more pirateish.
A new rug.
Some of the old fashioned looking maps on the wall (the ones that are brown and yellow shades) or some ship prints
A pirate chest toy box