Assalam o alaikum Sheikh ! As birthdays are haram so selling products related to birthday , but my father sells these products related to birthday in his bussiness. Should I help him in bussiness or excuse myself , also I fear that if I excused to help him it will rip apart my relation with father

Do not help him in the birthday goods themselves, for there is no obedience to a creation in disobedience to the Creator, and Imam Ibn Baz answered a son in exactly this case that he withholds his help in the forbidden thing and asks his father's pardon for that one thing with gentle speech. Help him in whatever else that is lawful, keep the birr and the good treatment, and you are not responsible for his reaction.