Is the reason for the son's affairs being hindered, despite his righteousness and devotion to his parents, his mother's prayer for his return from Canada, and does she owe expiation for that? And what can the mother do to relieve her son's distress and help him obtain citizenship in France?

Your previous supplication is not the cause of what is happening to your son. If what afflicted him is indeed an evil eye or magic, then its cure lies in legitimate ruqyah. You both should strive in supplication, asking Allah to facilitate good for him. Know that whatever Allah decrees for a believer is good for him. It may be that what you dislike contains good for you, as Allah Almighty says: "And it may be that you dislike a thing which is good for you and that you like a thing which is bad for you. Allah knows but you do not know." Contentment with Allah's decree is one of the qualities of believers, for Allah guides the heart of whoever believes in Him when afflicted by a calamity.