Step Parent Adoption / Is there another option

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I really need some advice in this.
My children are 17,15,14
12 months ago my eldest and youngest asked if they could change their names to my fiance we marry in July.
I said you need to spend time thinking about this and I will talk again next year about it.
12 months on neither see their Dad he is not stable or reliable and cruel.
My relationship with him was abusive and controlling but I have tried my hardest to seperate my dislike for their relationship this is not always easy.
Anyway he told my middle daughter by text as she asked him if he would sign the forms to agree to name change he said ok now they are ready to sign he says no way.
But if he wants to adopt them I will sign that.
Can anybody please explain to me why a Dad absent or not would say this.
My kids now want us to pursue adoption do you think because of their ages and my son hasnt spoke or seen him since Dec 14 Eldest not since around October 2015 bar Their Grans funeral but non of them sat near or spoke to their Dad at all.
Would we have any issues with this or is there an alternative. I do not think he will agree to anything he doesnt want them but his controlling side will take over and he will see this as a way to assert the control.