First of all, these are definitely my own opinions but.... I think Tsutsumi was called by his father at the station when he was about to leave trying to get him to stay.
*****spoilers***** but my opinion to the question
Second, when nakamichi decided to give up and die, Tsutsumi senior appeared and was disapproving, kind of saying "dont give up, idiot." Third, tsutsumi Sr.'s apppearance after Junior was hugging Nakamichi was probably his way of saying..."im glad youre okay. I didnt leave you alone. It's okay to love my son, too."
Im sorry! I had spoiler alert put at the top and when it went through, the spoiler alert had moved to the middle of my paragraph....
I think that's part of the reason. But mostly Nakamichi has regarded the son as an indispensable part of his life. They share joys and sorrows along with the memories of the director.
can someone explain, is he with the son because he is afraid of being alone i mean he said he could never love anyone as much as he loved the father and while he was being saved he saw the father.