NBA great Julius Erving — Dr. J — believes that his missing son Cory, 19, is still alive, although he concedes that the teen was probably on drugs at the time of his disappearance over Memorial Day weekend. On Monday’s edition of ABC’s “Good Morning America” (Erving made a similar appearance on NBC’s “Today”), Erving said, “I don’t think he was clean. I think he was still participating.” Erving spoke from a TV studio in Philadelphia, home of his greatest basketball success. At a subsequent news conference, Erving revealed that he had learned from a friend of his son that Cory had been indulging in marijuana and “roofies” — the slang term for the illegal sedative Rohypnol — the night before the disappearance. Dr. J appeared at the Philly City Hall with his other sons, Cheo, 27, and Julius III, 26, who choked back tears as they described their brother and pleaded for him to come home. The national hotline number to help find Cory Erving is 888-609-2529.