well...if your anything like me...an important word is budget!!!...i also am doing a pirate bedroom for myself, and this is what i've come up with.
mind you...you might have to be slightly crafty.
i bought white sheets (flat) and put in grommits at the top. i attached to a pole at top of window with sisal rope. looks very "sail-y", maybe you could also tea stain??
a captain's bed would be cool (from anywhere), must ding it up. (used look)
lantern / sconces to place on the wall.
buy lots of chests sm-med-large. ive got several sm-med decorating the top of my dressers. my wallymart has them for $5-10 each. the large one i'd place on the floor. you might even find a beatup antique chest to place at bottom of bed. (mine hold all my pirate clothes)
to fill the chest...beads of course, fake rinestones, lava rocks spray painted gold are a good filler. also, hit up your local sal. army/goodwill. you can buy silver, bronze, and gold "treasure" (plates, goblets, candlesticks) VERY cheap!
and so you're not having to fill the whole box, place something in the bottom to take up some of the room.
if he has pictures to hang up, i took several bamboo sticks and lashed together with sisal rope to make a frame. could use driftwood if you can find the right pieces.
get a book about rope knots. make knots and display on wall.
dont forget old wine/rum bottles (...and parrots & monkeys) to lay about. could also get candle and melt down to bottle. make sense?
hope this helps!