He was there to pick up his son I believe. They had joined custody I think. So he had every expectation and right to go to the front door and pick up his son. Ex wife and stepdad are wrong here from the looks of it. But the father should have left when the other guy came out with a gun.
But if someone tells you to leave their property, and in so breaking that agreement, then it is automatically a legal issue and police and the courts need to be involved. Fighting the other parent isn't going to help your case, just jeopardize it.