what to do about filing charges

Since the person who did the assault was a police officer, it would not be wise for you to report the incident at the police station he worked at. You can either go to the other police station to file a report, or to go negotiate with the police officer threatening him to pay for the damages he caused or else you guys would report to the higher ups. Of course if you guys don't want to meet him in person, then you always report directly to the higher up because they have to power to punish. Of course at the same time of doing that, you should consult a lawyer directly on what is the possible compensation. They know better than anyone else.

you need a medical report about the injuries he suffered , this is the evidence that you may present in court and also the statements from the witnesses, you need a public or private attorneys to handle the job, and also they are the one that will going to evaluate your case, it is a long due process but it is the procedure, and if they find you have a strong evidence then it may be file in court. Usually a public attorneys take a free of charge for the cases they handle.

There are certain charges - medical bills for example - that still may be recovered in the civil suit. However, depending on where you are depends on the cap that is has been placed on the amount. If he is found not guilty (which if they have all of that evidence I doubt that he will) it may be harder for you to sue in a civil suit.