I don't think he forgets Etienne is his son, but instead wants to run away from reality. At least that's the impression I got from the story. He doesn't behave like someone with neurological conditions, more like a scared man.
I mean, it's not like I don't understand him. The queen abused him and killed people close to him, he got really traumatized, I don't blame him of having fear.
But he's so scared of her that he tries to run away to the fact that he used her status to become king in first place, that he has a son with her that didn't do anything wrong and that he had and still has power to do something about this situation, but doesn't do because he's scared. He doesn't own his mistakes, doesn't fight for change and don't even think about other people suffering.
About Rishar, I think he perceives him as an escape from his reality. Rishar is like his salvation that, not only remembers him of his happier days, but that also has the potential deal with the Queen and her faction for him.
Delusional people normally bend the reality. If he was delulu maybe he would even forget Etienne and the Queen, that is the source of his trauma. But he's really aware of them and hate himself for it.
In the deep, I even think he knows that Etienne suffers in the queen's hands (Since Rishar told him when they where kids that the queen was beating him), but in his escape from reality, he just ignore it and convinces himself that he's right: The Queen is the only villain, he's the victim, Etienne is her son so he's like her and Rishar is his hope.
That way he doesn't need to feel guilty for running away.
In my opinion, he's just an egoist and coward man....
But maybe I'm wrong hahahah is just my opinion really.
That's it, sorry for my gigantic text ⁄(⁄ ⁄·⁄ω⁄·⁄ ⁄)⁄
The thing about Etienne's father is that he's not an evil, novel villain like the queen, but he's the kind of man we see it everyday: a coward, frustrated and neglectful father, who convinces himself that he's the only victim of the circumstances and ignores his children suffering even if it's right in from of him.
This makes me hate him so much. I mean, he's not even that bad, the queen is way worse, but he's gonna have to lick Etienne's feet for me to even consider forgiving him hahahahha