My son has had racial slurs hurled at him the entire school year, isn't the district responsible for providing an atmosphere free from harassment?

To a degree, there is some responsibility here, but you and your sone (who certainly should not have to put up with this sort of treatment) need to be dogged in letting the powers that be know what is going on and asking for help. If you don't do that, you can't really expect much to happen. Sorry. Good Luck.