Solution:
If he is hyperventilating from an anxiety attack, the simplest solution is to have him breathe into a paper bag. He will rebreathe some exhaled CO2.This will increase PaCO2 and trigger his normal respiratory drive to take over breathing control.
* Please note this will not work on a person with chronic CO2 retention, such as a COPD patient. These people develop a hypoxic drive, and do not respond to CO2 changes.