and in baltimore sun interview dad says he didn't have a problem with the cops
Irrelevant to the point. The point is, (and let's say the parent is 100% at fault) that he was arrested for assault on a police officer during an altercation with a private security guard, based on the private security guard's day job. What would the charge have been if the private security guard had a day job as a dentist?
Just because a cop is "on duty" all the time doesn't mean that charges related to laws regarding police officers applies to them all the time. If I have a neighbor and we get into a physical altercation over whose dog pooped on whose lawn, if he is a police officer, he shouldn't be allowed to decide to call up some buddies and have me arrested for assault on a police officer. Even if I was 100% at fault, it would simply be "assault".
If the cop involved in this was officially on duty, he should have been in uniform, not in a "security" polo. I've been to many .gov held public meetings where they decided they wanted Federal or State LEOs there as security. They all wore they're official uniforms so you knew exactly who they were and that they were there on official business, not moonlighting as private security.