If the gray-haired gentleman is the Simulator of our universe, then presumably he could demonstrate this by allowing you to "view" the simulation, in particular your decomposing corpse, grieving loved ones &c. Also, he could further replay to you events from the past: events you remember vividly, and perhaps some historical ones too. And also, he could allow you to spectate on the ongoing simulation.

Of course, he would no doubt have to provide a considerable number of bits of information in order for the Afterlife hypothesis to become more likely than the alternatives such as dreaming, drugs, major brain malfunction, &c.