Is he really a Rabbi or are you just being cute (in your mind) because he's Jewish?
Do you think it's a put down by calling him a Rabbi? I mean, what's the point of addressing him like that, otherwise? Because it's not cool to write, "Hey, Jew"?
A Rabbi is a learned individual, so if you think he's really one, pay attention to what he writes and take heed. If a Rabbi thinks you're a piece of human excrement, he wouldn't say that to be glib and there's a good possibility he's correct.