Spiteful ex

HI ladies.
I am at my wits end with the father of my sons. We have no contact with him that’s his choice. We have a 7 year old and 4 year old.

We broke up when I was 2 months pregnant with our youngest son, because he was violent, I found out he was using ice and doing criminal activity.
When we broke up I never stopped him from having contact with our son and still invited him to ultrasounds for the new Bub none of which he showed up for... he then got a new gf within 2 weeks of us breaking which I was fine with my priorities were my kids not him or his choices and then we didn’t hear from him for a few months, he contacted me when I was 5/6 months pregnant to tell me he was expecting with the new gf and I wasn abused by her telling me she hoped my baby came out ‘retarded’ and that neither of my kids were my exes *roles eyes*. I ignored and moved on... have since raised both boys on mum own and I’m very proud of my efforts and believe I’m rising wonderful Kids with bright futures.
Which brings me to today... he has since had 4 more kids to two different women and we have had absolutely no contact, he has been in and out of jail and that’s his choice of lifestyle....
His mother ( I barely speak to her, she is a drug user also) informed me he has change s his last name legally, which child support confirmed to me even though he doesn’t pay any child support he just has a debt accumulating.
He is only on our eldest sons birth certificate, so I need his consent to change his last name.
He got his mother to message me a few months ago to say he wanted to relinquish all rights and wanted his name off the birth certificate so I said for him to lodge it with the courts and I would not contest... I then because he claimed he didn’t have a way to the courthouse mailed him the changes on name forms for him to sign and have witnessed, he wrote *FUCK YOU* on the forms and sent them back... I just left it... now today I messaged his mother to ask if I could please send the form s to her addreess to have them signed as now that’s my son is 7 he can read and we have the commemorative certificates framed in my house he has questioned why his name is different to mine and his brothers, I explained the best I could... it left him confused and upset as he is still not quiet sure why his dad decided to not be with him anymore even though I do my best to explain gently and I don’t want to raise the kids in a house where one parent trash talks the other despite whatever differences I have with the ex....
Anywho I got A response saying that he is refusing to sign the last name change despite him not even having that last name anymore, I feel so sad my son has a last name that no one else has... no family nothing. All the exes other kids have his new last name. I just wish he wouldn’t worry about hurting me and just see what’s best for our son.... his excuse was ‘IM DOING NOTHING TO MAKE YOUR LIFE EASIER, THE BOYS WILL FIND ME WHEN THEY ARE OLDER’!!!
Will a judge grant me the change of surname as neither parents have the last name used anymore.
I will be seeking legal advice I’m just so shocked and Annoyed he won’t do this especially since he hasn’t seen him in nearly 5 years!!!!!