I wonder what the incident was that caused him to quit studying law.
Maybe it had to do with another illegitimate child, like a court case he was working on. Or he knew someone whose child was taken away just because having a child young and out of wedlock tarnished someone's reputation. Maybe the incident had something to do with Momoko's adopted mom dying.
EDIT: Look at chapter 7, page 20. It looks like he's looking at Momoko's mom. If so, that means they went to the same high school.
@Umesan
She said "he sounds like mom," meaning he feels like children should be protected by society instead of ignored / hidden for reputation's sake