YutoBK

I wanted to write exactly the same thing. He will be missed.

It is actually not outlandish, it is well explained in the episode. He actually tried to go to the police at minute 14:26 of the episode. It is only one sentence, it is very easy to miss. I missed it the first time as well. Anyways, I explained how the whole situation escalated in another comment to OP as a spoiler below.

Actually this is well explained. So he started asking Lion where his parents are. Lion tells him he wants to pee, so he goes inside his house to pee. Hiroto then knocks on the toilet door to ask Lion whether he was done peeing. Lion comes out of the toilet. Hiroto tells Lion that they should go to the police. Lion starts running around, causes a whole scene and then tells Hiroto that he will start living with him. Then Lion gives Hiroto the phone where it says "I will leave the rest to you". This is the same thing his sister told Hiroto years ago, so he figures that the kid was his sister's. The kid also uses mayonaise for everything, just like his sister. Even in food that normally does not pair well with mayonaise. After he sees the bruises on the kid, he knows, as a social worker, he cannot get the kid to the police as they will return him to his guardian who most likely is physically abusing the kid. You should re-watch the episode, because many of the finer details are not obvious from watching the first time. I watched it again for fun and then saw new details appearing left and right.

A journalist who is investigating a missing person case surrounding Aoi and Lion according to Japanese Wikipedia.

Almost guaranteed that it is his mother. Because the lady at the bridge was in Yamanishi prefecture, where the police station was stationed and also where Aoi Tachibana lives according to the cop that looked her up in the database. Also, Osamu Mukai (the actor) is called Shogo Tachibana in the series and he was filing a missing person report for his wife and kid. Also Machiko Ono plays Aoi Tachibana and I am certain that she was the actress at the bridge that tried to strangle Lion.

The theme song is already out: Vaundy - Fujin (風神)