That kid does look like a younger version of Shin, but I think he is blonde, so it could be Nana and Nobu. When Nana ask about lil Ren, Takumi just hang up on her and didn't answer, so that why I think it her and Nobu. I think Takumi still bitter about Nana and Nobu even though he accept the kid.Also if he was Reira child then why did she said this line in the manga "That boy with the same name as Ren...."