If I were the father I would go to the management in person with my son, explain the situation as you did in your posts and ask what ordinances were violated to warrant the warning of possible eviction. It may be a plus if he can get the responding officer to accompany him and his son.
It is possible someone complained simply because they assumed something more was going on given the presence of police. It is now up to the father to make it known to the management this was a friendly service the police were called upon for.
JMO

