My sympathy too. I am sure you are much more angry about it happening to your son than you would have been if it had happened to you. To some extent I have become spoilt by living in Cologne. Of course, one has to constantly put up with rude and anti social behaviour on public transport (I prefer to cycle if I can), but actual beatings are very rare.
I think the deterrent here, which seems to work better than the British system, is that the police usually arrive at the scene very quickly, so there is a larger chance of the assailants being caught. Severe sentences (by British or American standards) are almost unknown.
I should add that I have no sympathy at all for those who go out and beat others just for their own enjoyment. Regrettably, these people have always been around. I always enjoy stories about these oafs getting their comeuppance. I found Dave the Gnome's story very gratifying.
I hope your son recovers very quickly.