It was stated that the son was the embodiment of the dark side, but then the father says that the son was falling to the dark side which suggests otherwise. To me, it seemed as if the Mortis trilogy was trying to depict the son and daughter as representing emotions, with the son representing dark emotions, such as anger, rage, jealousy, and hate, while the daughter represented emotions such as love, serenity, happiness, etc. With the father, or course, being the balance of these emotions and keeping them under control.
However, the father did claim to try to control the dark side in his son, which was a futile attempt and something impossible to do, as the dark side shouldn't be used because it's like a cancer to the force (as Lucas once stated), and no one is capable of using it successfully without being corrupted by it.
That was my POV after watching the arc, but the EU changed that and didn't leave my opinion it up for interpretation.