The term "Son of Man", as used in The Waste Land, implies a sort of weakness: You are only human. You know only what your senses can tell you, and that's not much.

This is a lead-in to the subsequent offer of shelter under the red rock. My best guess is that Eliot intended this entire stanza to be from the perspective of a supernatural being - perhaps the same spirit(s) whom Madame Sosostris intended to contact with her Tarot? An alternative possibility is that Eliot intended the red rock to represent a teacher or mentor who sheltered him in a rough time.