Changing my illegitimate son's surname

Dear Sir/Madam,

My son is using my maiden surname including my maiden middle name. But when my son had been transferred to a different school my son's grand mother used his biological surname without my permission. The guardian is my ex's parents. My ex and I were never married but he acknowledged my son on his birth certificate. My ex has never had parental responsibility to my son due to his parents interference. My son is now 15yo, I'm now married to a different man and my ex is also married. My son made his decision that he wants to use my husband's last name. His biological father agreed to our son's decision. My question is, can my ex sign an affidavit that he permits in changing my son's surname or my husband and I need to adopt my son? Please advise. Thanking you in advance.

Kind regards,

Mrs A

I don't think it will be that easy.
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