How can one help a man over seventy years old stop boasting about his past misdeeds, especially since previous attempts have failed to deter him from repeatedly mentioning these heinous acts?

Speaking about a sin after committing it is a serious transgression added to the sin itself. The perpetrator must repent and abstain from it, especially as they age, for Allah the Almighty said: "Did We not grant you a life long enough for whoever would be reminded therein to be reminded? And the warner came to you." And the Prophet, peace and blessings be upon him, said: "Allah has left no excuse for a person whose life He prolonged until he reached sixty years." The advisor must continue to counsel and remind him of Allah's punishment, and utilize available da'wah (preaching) methods.