Great question, especially in light of the fact Sonny said this to Mike in the presence of Tom, who later refused to accept a letter from Kay to Michael because he believed if he even took the letter they could prove in a court of law that he knew Michael's whereabouts. My guess is he let it go when saying goodbye to Michael but later told Sonny thathe could not get word to her, and obviously he didn't because when Kay showed up unannounced at the mall she was complaining that no one had returned her calls or otherwise responded to het repeated requests about Michael.
It odd too that Tom thought he could tell her that Michael was "all right," but nothing more. If Kay had been subpoenaed and was forced to testify she would have had to tell the court that Tom Hagen admitted he knew that Michael was "all right."
Such an admission against interest would be circumstantial evidence to show Tom did know where Michael was, or at least knew hw to contact someone who did.