He probably had no idea how else to react, and so was already in the habit of saying "God bless you" anyway...At least I know that's how it would have been for me when I was a believer.

Since we are our own gods, you could see it as him wishing you well.

I usually see just a random blessing by a Christian as something they do just to have something to say. For example when you sneeze they also say “god bless you”, it doesn’t mean much, so I don’t take it seriously. Live and let live.

Good for you, you left him in the dust