His face would probably go through a series of different expressions before it settles on one. I like to think it'd be amused exasperation that wins out in the end.

[He pours himself more milk, then shakes the carton slightly in Peggy's direction, a silent offer to top off her own glass.]

Can't be any worse than that time I ended up calling him Captain Small Ass.

[His expression may be a little too innocent as he says it.]