It totally makes sense that Alex and Henry share protagonist duties in the movie, since it would be shame to waste Nicholas Galitzine in this film. However, the fact that everything is from Alex’s perspective in the book changes the story so much. For instance, when Henry ghosts Alex after he’s about to tell him he loves him – that is BRUTAL in the book. Simply because Henry just disappears from the narrative for about 20 pages or so! It’s heartbreaking.
The lack of Henry’s POV also makes Alex seem more of an active character, since he’s driving forward all of the action in the novel. This works quite well given that Alex is supposed to be an aspiring politician and a bad boy teen idol, of sorts. Henry, meanwhile, is more mysterious and guarded, and as a reader you almost fall in love with him as Alex does by learning bits and pieces about him along the way. In fact, if you don’t feel protective of Henry by the end of the book, then I don’t know what to tell you!
Conversely, in the movie, a lot of the later action – in particular the email hack – occurs though Henry’s eyes, which is very different from the book.