I don't think he actually has another son. I feel like it's a protoge that was like a son to him, but got in with a wrong crowd.
If it actually turns out to be his biological son, first of all
, second of all, they better not stop at one if they are gonna go down this path. I want a whole bunch of kids he sired with other women before Yukihira's mom tamed Ashura.
The one thing that really has me bothered because of this chapter is now that Joichiro has lost to "Saiba", Yukihira is more than likely gonna a have to face him to get revenge for his father. If they do go head to head and Yukihira wins, won't that mean he's better than his father? Won't that render Yukihira's whole reason for becoming a better chef into a moot point? IDK, that's the only major gripe I have right now.