theres probably nothing you can do. the cops will take his side regardless of what actually happened.
Would'nt hurt to talk to a lawyer, you might be able to sue the asshole. I would at least file a complaint with the city, might not come to much but at least theres a report on file (ask for a copy). If enough people get harassed and complain, this pig will get into some kind of trouble. Better than just letting them get away with that shit.
cops do whatever the fuck they want, i have been arrested not read my rights locked up charged and screwed. some cops are just plain dirty and always remember the word of a cop is taken way more seriously than the word of a token teen.