maybe because he knew his son was being loved a lot by his mother in the countryside, far from his world, full of bad people. The other 2 didn't have anyone aside from him. But I think your comment is very relevant even to another thing: why when a couple from a manga (and other stuff) gets divorced, they don't keep in touch with the sons and daughters? I don't remember ever reading a manga with a kid going to spend every weekend with one of the divorced parents.
maybe because the law in Japan is a little different that in Brazil. I read somewhere that in Japan, it doesn't exist a shared guard and no visitation regime. If you want to read some more, here a link, in portuguese: http://exame.abril.com.br/mundo/a-guarda-compartilhada-e-os-pais-abandonados-no-japao/
=O thank you for the link!
You're Welcome! (=・ω・=)
Why did the father instead of spending time with his real son souichiro goes n live with the stepsons. And instead of being nice to his son he goes on being extra nice to his stepsons.