How do I go about taking someone off of my child’s birth certificate and how hard will it be for me to get this done?

My son, who is almost three and a half years old, hasn’t seen his real father in over 2 years. The 2 year mark was hit in May, 2020. I have given the father numerous chances to have a relationship with him but he never follows through with what he says. I haven’t heard from him in months now, and I haven’t voluntarily messaged him anymore because I feel I shouldn’t have to. If he wanted something to do with him, he would be making an effort. I would even just message and ask if he would send money every monty to help buy things he needs, and he won’t do that either. I am currently engaged, and the man I’m engaged to I have been with for over three years. He has been there and taken care of my son since he was 4 months old. My fiancé wants to adopt him. My son calls him daddy. And my fiancé and I also have a 1 year old together. I want us to finally be an official family and I need help doing this.