The old man is most probably the Ooka grampa, looking to take down his Jirokichi rival, using Conan for his own interests. Investigating Conan to find his weaknesses and use them against Suzuki. Ooka sounds like "Hawk". He's Momiji's and Saguru's grampa. Grampa needs help from PSB to fight the BO and Karasumas. He's close to Kuroda and his men. He asked Momiji & Iori to leave the phone in the carriage. He probably knew Renya long ago.
Make sense, but doesn't it make the story a bit stretched away from the plot? I don't think gosho will add more complicated episodes alongside black org arc since the plot related with bo is already big, considering the needs to explain the plot hole of the series for example tea party meeting. And based on this hawk guy's appearance alone tell me that he will bring along more than 5 chapters to complete his venture. But idk what gosho might plan. Let's just wait and see