Pull up your big boy panties and get over it. Even if you were supposed to be consulted, it's at worst a civil dispute. So you take it to court, she gets her hand slapped and told not to do it again, and the boy's name remains the way it is since by your own admission it's not reasonable to force him to change his name now.

But since you've already been told, by a lawyer, that she DID have the right to do that, it's quite unlikely that you'll ever get it into court or that she gets her hand slapped.

Might I suggest some counseling so that you can discover why this is such a big deal to you?