Is a son sinful for hating his father due to the father's ill-treatment of him since childhood, despite the son's repeated attempts to forget what happened and his inability to remove this hatred from his heart?

The right of a father is great, and honoring him is a cause for Allah's pleasure, while disobeying him is a cause for His wrath. Regardless of his state, honoring him is obligatory. Continue to honor your father and treat him kindly. You can ask him to be just or seek the help of someone who can advise him, and make a lot of supplication for him. Do not succumb to feelings of hatred towards him, and do not refrain from visiting him because of them, as these feelings will not harm you if you continue to honor him. Remember that reciprocating evil with good brings affection, and the father is "the middle gate of Paradise."