That sounds like the typical lines used to blame a victim who was abused or otherwise assaulted.
The question isn't what he was doing, it's what those 13 year olds were doing. Why were those children even out at that hour in the first place, other than refining their violent criminality skills?
Worst case scenario, let's say for the sake of argument that they actually encountered him trying to steal one of their cars. They still had no right to assault him like that.
And that's just a hypothetical, because we already know what the situation was, and it was they who were trying to steal a car, and he who got in the way of their fists and kicks.
There is no excuse or defense for what they did. The only thing we can hope for at this point is that they get exactly what they asked for.
Commit a crime, do the time.