There are a handful of States have laws saying that only a PE may provide expert testimony in a court of law, at least insofar as claiming to be an engineer or engineering expert. Some liken it to "stamping". I don't know how it works in North Carolina. The NC Board must've felt secure enough in their laws and/or regs to send him a warning letter.
The fact that he claims to be doing it pro-bono is a distraction. It's a spaghetti argument that if he isn't offering services in exchange for money then he isn't practicing of engineering. But IIRC some States even have regulations about free work and undercutting services. Again not sure if that applies in NC.
Having said that, the precedent set in Järlström v. Aldridge, a.k.a. the Oregon case, means that the <airquotes> retired engineer <airquotes> will prevail in his suit.
OT: In my humble non-legal opinion Järlström was wrongly decided. Imagine if a Juris Doctor (person with a law degree) who isn't a member of the XXXX Bar then claimed to be a "lawyer" in an official government proceeding in XXXX. That will usually get the JD a nastygram from XXXX's Bar. And I can't imagine a judge condemning the Bar for doing so. But apparently if a degreed engineering lacking license, claims to be an engineer in a government proceeding and gets a nastygram then its a first amendment violation and the regs needs to be rewritten.