Wouldn't cross my mind to do anything unless the following was the case:
-he looked old
-it was really HOT

It's pretty common for elderly people to not register heat... they can be very susceptible to heat stroke and dehydration for that reason. If that were the case, I might stop and ask if he was okay/would like a drink, or give the police a call just as a precaution. Otherwise, I'd just keep driving.