I would prefer when me and DP get married if we all ( me him and DS) change our last name to something completely different that we all like. To me this is more romantic as it feels like we will both be coming together in marriage and starting a fresh with a new name that reflects our family.
DP isn't writing off the idea and has said he will think about it. I think if he wasn't taking outside views into consideration he would say yes but his concern is probably more to how his parents may react (I'm guessing as he's never shown any attachment to his last name)
Anyway our situation aside what are your views on this