He ...... in wearing the old fashioned coat in spite of his wife's disapproval.

He insists in wearing the old-fashioned coat in spite of his wife's disapproval.
In this context, "insists" is the most suitable verb to describe the man's determination to wear the old-fashioned coat despite his wife's disapproval. "Insists" implies a firm and unwavering commitment to a particular course of action, which aligns with the man's behavior in the sentence. Therefore, "insists" is the correct choice to complete the sentence.