Quote Quoting JulieN
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His father wishes to change my sons middle name to his new wifes middle name and his last name I don't believe his wife has any business changing my sons name to hers

Ah - a vital point was missing from the original information.

In THIS case...I cannot see him being able to change your son's middle name.

But he generally will, in all likelihood, be able to have your son's last name changed (or at the very least, hyphenated to include his last name).

(Please bear in mind that this isn't about his wife - it's Dad who wants to change it, and the matter is between the two of you, not his wife)