He's a fool.
The reality is that he's human but he's not thinking of it like that. He's only stopped on the way home because he noticed a girl. Only stopped because she was surrounded, being dragged away, because there were hands on her jeans –
Only stopped because he noticed too much and was incapable of so much, and here was something right in front of him.
Only stopped because his sense of justice jumped the queue and launched before his sense of… well, sense.
And at least the element of surprise is on his side, because he's punched the first person in the jaw before they react to him.
Except now he has three people left, and they've still got a hostage.
And punches, even from a lean teenager, are ineffective when he's outmuscled and outnumbered.