Is it obligatory for a son to return to his home country to care for his sick mother and fulfill her requests, leaving his job, despite knowing the security risks he might face, and his mother's rejection of other solutions such as hiring a maid, staying with her other children, or moving to live with him in his country of residence?

You are not required to return from your journey. Most scholars permit a son to travel for trade or similar purposes without the permission of his parents, provided there is no danger in the journey or residence, and there is no fear of them being lost in his absence. Nevertheless, you must honor your mother, treat her kindly, strive to please her, and convince her of your need to remain outside your country.