What should a man do who married a relative sent to him while he was abroad, only to discover she had a birth defect, and he has not felt any love towards her after years of marriage, and he has a son with her, and he wishes to marry another out of fear for his religion, knowing that his wife will seek a divorce if she learns of his desire?

Allah has legislated marriage as a source of tranquility and affection, and has permitted polygyny up to four wives, in order to achieve great objectives, which may include a remedy for your situation. If the husband is able to provide financially and act justly, then the matter is permissible, and may even be recommended or obligatory. The wife's consent is not a prerequisite, because her refusal might hinder legitimate Islamic interests. Your effort to persuade her is commendable, but if she does not consent, it is not obligatory. You must marry to protect yourself, even if it leads to divorcing her, as long as you fear falling into what displeases Allah.