Welcome back! Wish I could help, but I don't know of any literature dealing with the specific topic of anxiety in middle-aged men. Google the topic and you may get info that you can print out. There are textbooks for practitioners that deal with onset of anxiety in middle-aged adults of both genders.
Maybe he needs to hear from a trusted Dr. or friend. Would your wife be able to speak about the matter with her father instead? Or MIL or your wife's other siblings. Maybe a family meeting to discuss the matter is warranted. In any case, good luck . . . I, for one, have dealt with stubborn middle-aged men of my own. No easy task