How can one deal with a mother who refuses her son to wear the Arab thobe, and wants him to adhere to a specific attire that she deems appropriate, and does what the mother requests have a legal basis in Sharia, and is the son sinful if he displeases her by refusing her demands in this matter?

There is no harm for the questioner to speak with his mother with politeness and humility, and to explain to her the harm caused by her interference in the details of his personal life. If she understands, then praise be to Allah. If she insists, then he is not obligated to obey her except in matters that benefit her, and it is not her right to compel him to wear certain clothes or eat specific food. This falls under the statement of Ibn Taymiyyah that parents do not have the right to compel their child to marry someone he does not want, and that this is not considered disobedience. It is better for the questioner to obey her in matters that are not sinful, even if there is hardship that does not amount to harm.