This quote comes from the movie ‘Red, White & Royal Blue’. The movie is based on a novel of the same name written by Casey McQuiston. | Red, White & Royal Blue is produced by Amazon Studios
Movie vs. book
In the novel Alex is not that kind of a person that would be confused about which title to use with Henry. He does use a lot of funny alterations of his title when writing emails to him, for example: “His Royal Highness Prince Henry of Whatever“.