If it’s in the context of him performing his duties, then Rabbi. For example, I wouldn’t have a problem with the parents of pupils calling him “Rabbi…” during PTA or in other school contexts.
Sounds like if they met in the grocery, you’d be fine if they called him Mr. I disagree.
Rabbi. They’ve earned it.
You do realize that semichah entails a test on certain limited portions of Shulchan Aruch, some of which aren’t very relevant to common shailos, and that the fellow may have forgotten most of what he learned.