that a child should always have their biological fathers last name if the mother knows who the father is. it doesnt matter that the father is not a part of the boys life at all, i think family names should never be changed you will always be your fathers blood no matter who adopts you or who you marry. i personally regret changing my last name after i got married, i will be changing it back to my maiden name actually. i dont know...thats just me, i dont believe in changing last names of kid even if you adopt them or changing you last name to y our husbands either. and also in this case, the fact that ther grandparents are active in the boys life and that they rejected their son for the way he is also should make you want to not change it, it would be extremely insulting to them in my opinion, and its not like the boy doesnt know he has a different biological father so he will understand why he has a different last name, i wet to school with tons of kids who had sibling in school with different last names and it was never an issue, no one asked or cared everyone had divorced parents and step sibling even in elementary school so they figured that was the reason for the diff last name.