I have a Off Track Thoroughbred here who has a clicking/popping hip. His popping is no doubt from old racing stress, however he is 100% sound to ride, he has been over jumps, on extensive trail rides etc. I wouldn't push him into eventing, racing, barrel racing, high jumping etc but he is sound and may never have a problem with any type of riding.
With your younger horse its much harder to say. He could have done something as simple as fallen in the pasture six months ago being a young boy and knocked himself out of alignment! I would look into a vet check and a chiropractor visit before making the "he is useless" assumption judging on someones guess no matter how reputable. A great natural barefoot farrier can do wonders on funny joint clicks and pops also.