Be careful! His biological mom may not be in his life at the moment. However, you are not married to his dad. You, for all legal purposes, have no claim on him. You may love him but bottom line is you have no legal claim over him. What will happen if/when you break up with your bf? I don't mean to sound negative but there is a child involved and you should be prepared. Unless there is some way you can adopt him, he can be completely stripped from your life and there would be nothing you can do about it. An innocent child would be hurt right along with you. Also, if you were to try to adopt him, his biological mom would either have to be proven unfit(if she hasn't been around, it can be proven) or willingly give up her rights. I don't know how old he is, but if he is a teenager and his biological mom shows up, i doubt very seriously if he'd willingly call her mom anyway since she's never done anything for him. However, a nickname/title should be secondary to the feelings this boy has for you. What would happen if you were stripped from him? Right now, you are a glorified babysitter as far as your rights to him go. I think it's important that he understand that you are not his real mother but you love him as a son. A nickname is just that, a nickname. It's the emotions behind it that are important.