Whose name to use on document?

My first wife deceased when my son was of 4 months now he is of 7 years. I got married living & together with son. Now many times i gets confused while filing a forms of school and many other places for my son. Question is which name to be used whether biological mother or his step mothers name as a mother. As Birth certificate bears his biological mothers name. please guide me legally which is correct way to use to avoid future complication/disputes.