I think a legal name has to be done through paper work (birth certificate or name change) but you can call your baby to others whatever you like. I'm not sure when it will come into play but it would eventually when they have to match name to social security number, birth certificate for things like school enrollment. Truthfully, I'd imagine that they are pretty savvy and know that a lot of parents would like to get benefits and will decline saying they have an involved partner in order to do so. Would it drastically change your benefit if you listed his income with yours?