That is odd, to just stop. Does he ever go through the motions of TRYING to crow (but no sound comes out) or does he never even make an attempt???
I know lesser roosters will sometimes not crow if a dominant roo is around, but that doesn't sound like your case. Your rooster's issue has me baffled...
Lol, I would be pleased to have a rooster that didn't crow! though I agree it seems odd for a 6 month old roo to stop crowing, have you tried moving him away from the hens to see if he starts crowing again? that's if you have hens! is the other one you have crowing? maybe he is the dominant one, have you seen them fight at all? there is nothing you can do to make him crow just be patient if he is fit and healthy he will start again can you post some pics up of your birds, both boys and any hens you may have?
I can post some up. We are actually not sure if the other is a roo or hen yet. None of our babies fight at all and he seems to be in perfect health. I often go out there and sit with them while they free range around our yard. I'll get some pics up!