Adding My Surname to DS birth certificate

[mumof5cn]

I am registering my baby's birth next week and have decided to give her a double barrel surname (me and her dads) as we are no longer together but my ds has only his fathers surname on his birth certificate.

I have 2 questions:-

My 2.5yr old ds has his dads surname but I want to add mine to double barrel it as I know his father will not let me change it to solely my surname.

Although the baby and hopefully my ds eventually will have a double barrel name, will they be able to use just the one?

why are you giving dads surname when you are not together?

i have never understood this?

Because my son has his surname. I know he won't consent to me changing his surname to just mine and I don't want my children having different surnames when they have the same mum and dad

No, a double barrel is both names which are the surname but 1 can be dropped at 11, will he allow a double barrel