Netflix made a ton of changes to John Ambrose McClaren for the movie. For one, he's described in the book as being blond, which is assumedly why a different actor played him in the first movie. He also doesn't volunteer at Belleview with Lara Jean in the book. He's actually Stormy's grandson who comes to visit her while Lara Jean is volunteering.
So he and LJ hadn't been building up their rekindled friendship while volunteering, but actually through letters. In the movie, John Ambrose wrote her back and she stopped the written communication there. In the book, they continued to be pen pals.