This is going to sound odd but have you done anything new to the pond that may involve electricity? Any type of lighting in the pond? It is not necessarily unusual for a Koi to lose its color. You can try to feed him something that has carotenoids in it. This can bring back the orange color. I have also read that some Koi fish have problems with their swim bladders which leads to them constantly lying on the bottom of the pond. I do not know if this problem would suddenly set in or if it would be noticeable from birth.