Based on the scenario you described, the heat casualty does not respond to verbal stimulation or shaking but does move when you rub his breastbone briskly with a knuckle. This indicates a partial response to a painful stimulus.
- "A" stands for Alert
- "V" stands for Verbal response
- "P" stands for Painful response
- "U" stands for Unresponsive
Since he only responds to pain, the best answer to assign him is P (Painful response).