You may have the rare chicken who is going through a sex reversal. Usually, when this occurs, it's a hen that turns into a rooster.
The reason this is possible is because chickens possess both male and female sex cells. When a hen has a damaged left ovary, for some reason, it produces less and less estrogen, and soon the right gonad activates and starts producing testosterone. This cause the plumage to change into that of a rooster and the chicken may begin to crow and mate other hens.
It's not as easy for a rooster to change into a hen, however, but not impossible. If your rooster, when he was a freshly fertilized egg, was slightly chilled for the three days following fertilization, this favored the development of female sex properties. Even though he developed into a cockerel and he's been a rooster all these years, if something has damaged his gonads, thereby drastically reducing his testosterone levels, since his early egg development was subjected to the right environmental stressor, he may now be reverting to favoring those female sex cells in his body, and taking on a female appearance.
I'm not saying this is what's going on with your rooster. There may well be another explanation as centrarchid mentions. It's just an hypothesis given the interesting biology of chickens and what may be possible with the fascinating little creatures we love so much.
