D.K.
Why should he be respectful? Children learn to show respect when they are treated with respect, see all the adults in their lives treat each other with respect and see that behavior modeled continually. Locking him up or punishing him will not make him respectful, it will make him angry and resentful. That is normal.
He should not be afraid of anybody, including a person in uniform. I do not want to teach my son to be afraid and I do not see why anyone else would either. I want my son to know that policemen are there to help us, not to frighten us.
My son treats his teachers respectfully because that is how they treat him. They say hello, shake his hand and make eye contact every morning. So, he does the same.