Hmm, I once saw pics of a room that had been painted, then the owner used a big rectangular sponge to make block walls, like a castle. She even painted a mural that looked like the landscape as seen through an arched window and the places in the "wall" that had crumbled. If you're not a muralist, you could just paint the walls, then hang candle sconces or sconce-type lamps on the wall on each side of his bed. Other things that seem medieval: fleur de lis, banners (think knights in shining armor carrying colorful banners into battle), coats of arms, the colors purple and gold, swords and spears (maybe holding up a canopy-type thing at the head of his bed?), scrolly ironwork, heavy furniture with carving, trunks, tapestries, chandelier-type lights.
I like the stenciled griffin idea. You might look at a craft store and see what kinds of stencils they have that might fit your theme. It sounds like fun, and what a great sister you are to want to help your brother decorate his room!
oh I like the idea about the walls.. and the castle.. is there some kind of shimmer paint i could use for that like silverish.. he has wrotiron(sp?) bed fram already.. maybe I can find the candles on ebay.. love the suggestions
keep them comming