Would your son, and his friends who "witnessed" this incident, happen to have been ever so slightly intoxicated at the time?
Was the bouncer also one of those who agreed that he "assaulted" your son? And was the same bouncer also present when the police officer "punched and beat" your son?
As NazNomad has indicated, the "full story" is definitely not contained within your above post - is it?
Given that "Police Brutality" is a very dramatic and emotive expression, it would suggest that the Police have unlawfully, indiscriminately, and unjustly chosen to deliberately inflict physical assault and injury upon your son, according to you, right under the noses of all sorts of witnesses including friends of your son. Not very wise, eh?
"what can i do" you say? Well, I'd sit your son down, ensuring that he is sober, get him to look you in the eye, and to repeat the allegations he has made above - unless he possibly has changed his version of events? I should also try to contact the solicitor he had access to at the police station during his interview following his arrest and find out what his/her opinion of what happened is? After all, the solicitor would also be in possession of the police's side of the story, also perhaps that of the bouncer(s), and maybe even any other prosecution, or even defence, witnesses. It may give you a much clearer idea of what actually did lead to your son's arrest etc.
Good luck.