He just realized it at the end I believe
spoiler
but i think the father was really Shouji, but Ririko knew that the only person she could rely on after she got pregnant was Sentarou.
it was not like that. He realized that out of them three: sentarou, touma, and shouji, Ririko (?) knows that sentarou is the only one she can rely on. i think the father was shouji as well but with being reliable and responsible, the best one is sentarou and touma realized all these when he came to know that shouji was ririko's ex as well.
Nope the last part he just piece the puzzle together why Ririko need to meet Sentarou like right that moment. He realises Ririko probably pregnant at that time n want to meet Sentarou bcs he's the only responsible person out all of the three
Can someone explain the final part? So Touma knew all along that Sentarou was really the father of Meguru or what....?