It was moments like these that made Severus glad that he had the experience he did as a spy. It meant that while he did briefly hesitate and blink, he was able to assimilate what had been said without looking gormless while he was at it. The young man in front of him looked like Regulus and certainly sounded like Regulus but... Regulus was dead.
He was momentarily confused about the mention of Potter but then, if this was truly Regulus and not some bizarre charade, it would have to be James Potter. Regulus had never met Harry Potter. If this was Regulus.
"I see," he said slowly, buying time to think more than anything else. He made no move towards his wand - yet - but he did ready a spell or two anyway. "And why should I believe you?"
Even as he spoke, he reached out towards the castle with his magic, down the paths only the Headmaster had access to, trying to ascertain what was true.