Will Allah forgive the son who stole from his well-off father if the latter regrets it, and how can the son appease his angry father?

If you are poor, your well-off father is obligated to spend on you with kindness. If he refuses to do so, it is permissible for you to take from his money without his knowledge, only to the extent of your need. However, if he does spend on you with kindness, then what you took from his money without his knowledge is unlawful, and you must repent and return the money to him or seek his forgiveness.