Some reasons they passed by on the other is because they don´ t know how to help him or he could have a weapon on him. 

In verse 20, did the son apologize when the father went out to meet him? What does this tell us about a father's attitude toward a child's mistake?

The son did not apologize to his father until verse 21.  This tells us that his fathers attitude toward a child's mistake is that the father excepts his child for who they are.  His dad will love his son no matter what happens. 
 
 In versus 22-24, Why did the father do all these things for his son? And what would this do for his son's attitude?

The father did the thing for his son because he was happy his son made the right choice to come home.  These actions showed the son that he is still loved and welcomed in his family's home.  His sons attitude will be better towards his father.