If I'm not mistaken, I think a person holds the title "Congressman" as long as they don't go into a higher office (Senate, President, Governor, Ambassador, etc.).
See 12. Ohio election law makes it illegal to do so in campaign ads and literature.
If I see McEwen on the street, it might be etiquette-appropriate for me to call him Congressman. It is not appropriate OR LEGAL for the McEwen campaign to call him Congressman in campaign ads. It implies incumbency and gives a sense of some kind of entitled superiority, which is one of the things I find very troubling about McEwen anyway.
Reminds me of an exchange in the musical "Evita." Eva is touring Europe to drum up money for Juan Peron and is meeting with a cool reception. She complains to a retired Italian admiral that she is being called a "whore" even those days are far behind her. The retired Italian admiral observes that he is called "Admiral" even though he hasn't been to sea in years.