Put a fly sheet combo on him to reflect UV light, thus avoiding/significantly reducing bleaching. A very light sheet won't make him too hot, in fact if you get one that is made mostly of cotton and not those horrible synthetics (synthetic material is primarily plastic, which makes the horse sweat) it will make him cooler. Black/dark colours absorb heat very effectively, and white reflects heat, thus you'll be helping him keep the heat away ;)
Also, make sure you wash any sweat off him. Sweat is great at bleaching dark coats, so every time you ride him, wash the sweat off, and if he sweats in the paddock, try sponging him down to get rid of it.