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)

