Does your son have joint parental responsibility? If he was born after 1.12.2003 and the birth was registered jointly by your son and the mother then your son does have reponsibility. Otherwise, he only has it if it was agreed with the mother, or if he got a Court order giving him responsibility.
If he does have responsibility AND there is a Court residence order in force then the name cannot be changed without his agreement. If necessary, he can apply to Court for a prohibited steps order to prevent the change.
Also, if there is a Court custody order in force it cannot be changed without the Court's agreement.
However, he needs to think about the impact on his son, because one name change has apparently already taken place which the son knows about - will he possibly be confused or adversely affected if the name has to be changed back?