I get this with my partners dad all the time. He will pay the bill without asking me about it first. I cannot tell you how much this pisses me off. I have, with his other daughters husband resorted to heading him off at the pass, by paying the bill behind is back the last 3 times we've all been out. Its like an arms race.
If I were you, I would say something along the lines off I am so pleased....nice girl...making of him....how would you feel about this being my treat....
nice problem to have BTW