Indeed it was a good movie. I think maybe that kid wasn't his son. Because Don bachelor and all never had an sort of serious relationship. Don is aging, and possibly after a recent run in with past girlfriends having moving on tries to quickly grasp something out of desperation. He realises that his whole life has mounted up to nothing. Only when the kid runs away, he truly knows what his missed. A family and someone to grow old with. I think when he got punched in the face was a wake up call for him because it showed how old and vulnerable he is, especially when he saw the blood.
This is just one of the many speculation I have.