How can a mother help her son, whom she cursed with a lack of success, and who is now in distress and indebted, knowing that he has repented and mended his ways?

You made a mistake by supplicating against your son, for the Prophet, peace and blessings be upon him, forbade that. Repent to Allah and replace your supplication against him with supplication for his well-being, and do so frequently. If you can help him and give charity on his behalf, then do so, for that is better for you.