Baby's last name

I'm literally due with my second baby, my son's biological father has never been in the picture so my son has my last name. I'm in a relationship but not married with my second child's father, and even if I did marry my partner I don't know if I really want to change my last name. I would like to hyphenate the baby's last name but my partner only wants his, I don't like the idea of me not having the same last name as my daughter and my kids having different last names and I feel like using both our last names is a good compromise. He said before since baby is a girl he doesn't mind if we hyphenate it but now he has an issue with it, saying her name will be too long if we do that (we are using 2 middle names) and she "won't have his name". Should I just give up and use his last name or should I fight to use both?