Is the son obligated to fulfill his promise to his mother to leave his job and return to her?

You are not obligated to obey your mother's request for you to resign and return if there is no valid reason for it, or if you fear that doing so would lead to ruin. Fulfilling a promise is desirable, and its desirability is emphasized when fulfilling a promise made to one's mother. If you are able to return and obey your mother, then do so, for honoring one's mother is among the best acts of drawing closer to Allah.