My step-father was referred to the orthopod . with a painful knee . However , when they x-rayed it the knee was ok but his hipsocket was almost gone and needed replacing . His knee pain was due to his hip .
As has been said , and as a sufferer myself I know , keep as active as he can but not overdo it . I find I stiffen up and get more painful if I sit or stay in the one position for long , but also suffer if I've been active for a while too. All the best to your father x