A letter outlining the situation to his department will likely be enough. My Dad used to get letters sent to his deparment occasionally and each time no matter how stupid the letter was, he got counseled and the letter ended up in his record with documentation on any action taken.
He never got busted because the letters were generally stupid, but even though he always tried to be a "good" cop he did learn a few things from some of the letters.
For example, on one stop the driver was arguing over the ticket and my Dad could see that the guy's kids in the back were getting a bit anxious. So when my Dad finished all the paperwork and was walking back to his cruiser, he waved and smiled at the kids to try to leave them with a last impression of a smiling cop, not an argument. Well, the guy wrote a letter of complaint saying my Dad had flipped off his kids. It was stupid, but my Dad got counseled on the "incident" anyhow.
My point is that a letter stating the facts and then a calm opinion that the cop's duty is to protect and serve, not enforce some imaginary curfew, would probably serve your purpose just fine. If you get harassed later on, you'll also have the letter handy to send to the mayor or your congresscritter.
As long as you keep it polite and not too accusatory, the officer won't get too crapped on but he *should* get a course correction from his supervisor.
