Final answer:
The most appropriate way to refer to the elderly man is as Mr. Walters.
Step-by-step explanation:
The most appropriate way to refer to the elderly man in this situation would be as Mr. Walters. Using a formal title like Mr. shows respect and is considered polite when addressing someone. Using his first name, Rob, would be less formal and may not be appropriate unless he specifically asks to be addressed that way. Similarly, using the title Mister without the last name may be considered too casual.