What should the son do towards his father who refused to go with him to propose to the girl he wishes to marry, knowing that the father has forgiven him and is not against the marriage?

If your father does not object to you marrying the girl, then there is no harm in proposing to her without him. However, it is better to strive to please your father and convince him to go with you to propose to her. You can seek the help of some relatives or religious scholars for that. You must continuously honor him and treat him kindly, for his right is great, and honoring him is among the best acts of drawing closer to Allah. The Messenger of Allah, peace and blessings be upon him, said: "The father is the middle gate of Paradise, so either squander this gate or preserve it."