He is a poor son whose sonship does not make him desire to serve all men’s mothers.

The line uses “sonship” as a moral and spiritual identity rather than a mere biological fact. Fosdick’s phrasing suggests that if one truly understands oneself as a “son” (implicitly, a child of God and a beneficiary of maternal care), that awareness should expand into active compassion: a desire to serve not only one’s own mother but “all men’s mothers,” i.e., all who bear burdens of care, vulnerability, and sacrifice. The aphorism presses filial gratitude outward into social ethics, turning private devotion into public responsibility. It also elevates motherhood as a universal symbol of human need and dignity, implying that reverence for one’s origins is hollow unless it becomes service to others.