Won’t be allowed into my fathers life without providing a DNA test

My Dad, who raised me for 18 years will not allow me into his or my little (half) brother’s life again after I made an anonymous post about my stepmother on a social media platflorm, unless I give my Dad a DNA test to invalidate/validate his paternity. He’s been married for nearly 10 years and he still has some lingering feelings of resentment towards my mom. This situation is heartbreaking for me. Should I humor his DNA test idea? Should I give up on him?