From my POV, if that baby was Wataru, it would have been more illogical. Here are several points that I found out:
1. The year that baby boy appear was in 1986, that would make the baby boy 22 by 2008, Wataru is 20, his birth year should be at 1988.
2. I still believe Maya could still be Wataru's mommy, because how would an baby boy pop out out of now where? It is quite unlikely that Taiga will be Maya's son. I believe Maya will eventually be with Otoya and will give birth to Wataru by 1988.