A "Rabbi" is a specific kind of teacher, one who is certified to instruct in Jewish law. A "Rabbi" must received semichah (ordination) from a qualified yeshiva (seminary).
Since you are a Christian, it follows that you are not a qualified instructor of Jewish law. Therefore you may call yourself "Professor" or "Reverend" or even "Doctor" (but only if you have a PhD) but you are not a Rabbi.
Actually, he could call himself 'rabbi', but not 'Rabbi'.
"Since you are a Christian, it follows that you are not a qualified instructor of Jewish law."
Are you speaking of the Torah? Because if you are it is a requirement that Christians know that law very well. Now I know very well that most Christians are taught that the 'law' no longer applies, but that is not the first seduction planted in Christianity.
I am not a christian, I keep Torah. The Sabbath included and Yahweh's Festivals (Yearly Sabbaths). I try to keep all the Commands of Yahweh. I do not care that some Yahudim teach the Talmud as the final athority. I only go by the Scriptures, not "king jimmy" the pedophile or what some Babylonian foisted on my Hebrew brothers twenty five hundred years ago.
Was Jesus not a Rabbi?