'He's not as good looking as you'

I was at the gym today. I decided today would be a good day for a run on the treadmill. I only run on the treadmill. Don't ask me why, but running on the treadmill is pain free, while running on the track or running outside gives me shin splints. They hurt like hell. That's a story for another time though. (Actually that's pretty much the entire story. Not really that interesting....but still, a tad strange I think.) When I was finished I went to the water fountain and some man told me that when he saw me he had to do a double take because he thought that I was his son.

'Oh yeah?', I said.
'Yeah, only he's not as good looking as you.'

Now I'm 99% certain that he wasn't hitting on me, but it was kind of uncomfortable. What is the appropriate response to a statement like that? Seriously? Do I say 'Thank you?' Or do I say 'Wow, I'm sorry your son is not as attractive as I am?' I had no idea how to respond to that, so I just smiled and walked to the track to cool off for a few minutes. Or until I was sure that he had left the gym. I wish people would think before they speak. Seriously.