His Highness’s birthday

Assalamu'alaikum !

WHICH is correct:

a) His Royal Highness 81st Birthday, or

b) His Royal Highness’ 81st Birthday?

If both can be used, are they pronounced the same?

Phrase (a) is incorrect, because the possessive is not indicated. Phrase (b),“His Royal Highness’ 81st Birthday”, is sometimes used.

However, since “Highness” is not a plural noun, it is better to add an apostrophe ‘s’ after it and write “His Royal Highness’s 81st Birthday”.

“Highness’s” is pronounced with one more syllable than “Highness”: it is as if you are saying “Highnesses”.