Obviously the wizard didn't like the question that Jake had asked. Regulus' reaction was unmistakable. Which told Jake that Regulus was hiding something, and even protecting someone.
Jake looked up and gave the men who had brought the patient in a pointed look. "He might lose consciousness," he said curtly, then pried open Regulus' jaws and forced the stone down the wizard's gullet.
The stone had a way of finding the source of the poison, and would not stick in the esophagus - but slide downwards by magic until it reached the stomach.