First, on the question about the origin and meaning of the expression “the son of man,” I’d taken from Ehrman’s discussion the inference that he affirmed the now-outdated view (formerly held widely) that the expression was well established as a fixed title for an eschatological figure in ancient Jewish apocalyptic thought. He insists, however, that he accepts that the expression wasn’t a fixed title or well-known. He allows that Jesus may have coined it, or may have appropriated it from somewhere (he’s “agnostic” on the question, to use his own term). But, he also insists, Jesus used it to refer, not to himself, but to a future eschatological figure.
It’s this latter bit that remains, in my view, a problem, however. If “the son of man” wasn’t a relatively well-known title for a relatively well-known figure, i.e., if it was something of a novel, or at least very unusual, designation, then how were Jesus’ disciples supposed to get what “the son of man” designated?
My own view (and not mine alone) is that Jesus took an expression frequently used in Hebrew & Aramaic, “a son of man,” and adapted it to express a particularizing force: “the son of man.” And, further, I hold that he used the expression as his own distinctive self-designation, comprising what we can call a feature of Jesus’ “idiolect” (to use a linguistics category). This fits with the fact that the Gospels clearly all understand all uses of it as self-designations. So, either these writers all were confused, or deliberately shifted from Jesus’ meaning (thereby over-riding Jesus on the matter), or else the Gospels preserve Jesus’ use of the expression as his distinctive self-designation. Certainly, it’s noteworthy that there is no evidence in the NT that “the son of man” ever functioned as a confessional title, unlike, e.g., “Messiah/Christ,” “Lord,” “Son of God.” I’ve laid out my thinking further in my concluding essay in Who is This Son of Man?, eds. Paul Owen & Larry Hurtado.