Suggest Treatment For OCD

My son is 26 and just got back from a two year mission. He s been on meds since childhood but for some reason he has been troubled with thoughts that make no sense or do but ....I want to help him cope. He has OCD. He says he feels held hostage to thoughts that don t make sense, that he can t get these out of his mind. There is a range of many different thoughts. He seems to replace one with another and another, etc.