PoP, Sorry to hear about PoP Jr. Exact same thing happened to me when I was 19 y.o.a., not far down the road from Newcastle.
No money changed hands - I think the assailants were more jealous of my friend and I's lovely long hair and winklepicker boots (which made it very hard to run away). Swift dental work was required for me, as well as having a hand in plaster for 6 weeks (it having been kicked repeatedly while trying to protect my head with it - nice), and my friend required a new nose.
Felt a bit paranoid for a while after, but it soon wore off, as I'm sure it will for your son. I think it was more difficult for the people around, with feelings of rage etc as you have described, but you just gots to let it go in the end.
Years later we are able to chuckle about it and call it character-building etc etc, but it was a bit grim at the time I must admit.
BTW - Not sure if the Criminal Injuries Board still exists, but check it out - I remember that at the time, even though there was no prosecution (and the reality is that there probably won't be for you either - not to your satisfaction anyway), we were able to make a claim through this, and ended up with a nice pay-out a year later. Money might not be uppermost in your thoughts at the moment, but it's worth making a claim if you can get it.
Ah - just googled it - have a look here:
https://www.cica.gov.uk/portal/page?_pa ... ema=PORTAL
JP.