Is travelling and leaving one's mother, who refuses to travel with her son and asks him to return, considered disobedience to her, especially since the mother is well-off and has brothers who care for her, while the son fears for his children due to the deteriorating security situation and has no home?

You are not obligated to obey your mother by returning to the country where she resides, as long as doing so would harm you. This is because obedience to parents is not obligatory in matters that would harm the child, as mentioned by Ibn Taymiyyah. However, you must still be dutiful and kind to her within your capacity, as long as it does not cause you harm.