Most Helpful Guy
You don't understand because you are not a parent.
Is the boy's mother involved in his life? Dad doesn't the boy being interrogated by his mother on the subject of dad's new girlfriend.
The boy's mother is not involved? Maybe dad doesn't want the boy to get attached to you and then, if you break up, the boy hurts, too! Or the boy gets attached to you, dad wants to break up but doesn't want the boy to be hurt, so dad stays with you for the benefit of the boy.
Dating people with small children can be difficult.