If the builder is infringing on Fender's patents, trademarks and/or copyrights, Fender has every right to and should go after the guy. Without enforcement, nobody's ideas are protected.
Any patents or copyrights have long since expired. About the only thing I can think of that they might try make a "materially similar" argument for is a trademark assuming they've trademarked the headstock shape. Trademarks don't expire like copyrights and patents do.
FWIW, Fender will never take this guy to court. Because it's doubtful they could win on so weak a claim. However, that doesn't keep them from breaking him by repeated filings and demands for irrelevant documents, requests for delays to prolong the legal process, and similar tricks to skyrocket his legal fees and force him to settle. That's done all the time and has become a familiar tactic employed by IP attorneys.