What should I do about someone who asked me out?

I work in my dad's doctors office. I'm a college student. I've only been working there for a few weeks, but today the office got a strange phone from a patient requesting to speak to me. When I answered, he talked about how he remembered talking to me in the office, and then, much to my surprise, he asked me out. I don't even remember this guy, but my dad told me his wife just passed away and he doesn't think that the patients thinking straight. Other "sidenote," he's 67!!!! I'm a little freaked out. How do I deal with this? Going out with him is NOT an option obviously. I think I might write him a letter. I just don't want to make him feel embarrassed, especially since he's grieving the loss of his wife. Help!!!