It looks like Caspian wears elements of this outfit at different times throughout the film.
He wears just the shirt, trousers and belt when practicing swordfighting with Edmund on the ship (check out the set video at NarniaWeb). At another time, he wears it underneath a brownish velvet tunic. The best shots with the velvet tunic—which we can't post here for copyright reasons apparently— show Caspian, the Pevensies and and the crew in small boats, possibly approaching Narrowhaven. At another time, on the Dawn Treader, he wears the outfit with another layer—a reddish coat with gold trim.
He also wears the shirt at the end of the world—we see him rowing out amongst the lilies with Edmund, Eustace and Lucy in the trailer.
The maroon/purple fabric of the shirt has a subtle sheen to it and looks somewhat crisp—probably a cotton, maybe a cotton sateen. The body of the shirt is gathered directly into the mandarin collar at the back, and the sleeves are gathered into the armscye and into a small band at the wrist (less than an inch wide).
The front slit extends to mid-chest and closes with three purple looped buttons. A line of embroidery stitches in a darker purple thread border the collar and neck opening, topstitching the facing. The facing is stitched again in a simple running stitch about an inch out from the opening.
He wears taupe-colored trousers with two knife pleats on each side of the front, stitched into the wide, high waistband we can see in these shots—usually covered by his wide belt. In the latter image you can see the trousers lace at the back of the waistband for an extra snug fit—they seem to lace through two pieces of leather.
The belt is dark leather with subtle etched designs and gold rivets. It has a narrower leather strap that wraps, possibly from his left side, across the main belt and buckles at the front. He wears leather boots that come up a few inches below the knee.
When he wears his velvet coat, the belt is worn over the tunic, shirt, and trousers. The velvet is more brown than maroon. It wraps and buttons to his right side, Telmarine-style, with a shallow, rounded square neckline, his purple shirt and collar showing.
The sleeves are long and full, pleated into a long cuff (though not into the armscye). They're slit open the length of the sleeve, from armscye to cuff, at the thumb side of the arm.
The edges at the neckline, sleeve slit, and front closing are finished with a dark narrow binding.
Just three shots of this coat with the outfit. It's dark red, possibly a jacquard, though it's hard to tell, with gold trim up the front edges and the mandarin collar, forming frog-like loops below the neck (as in the closure, not the amphibian). It's sleeveless, worn over his velvet coat, purple shirt and belt.