Do not leave a letter. You will cringe for life. Since you know he likes someone, first wait and see if he uses this advice on you. If he does then you can tell him you like him. Otherwise it will be pointless since you already know he potentially likes someone else.
For future reference, if you don't know if they like you back or not. Just say it. I know it sounds crazy, but people are very roundabout. My first partner I just said "I like you, do you wanna go out" and they agreed. Recently I told a girl I liked her, she didn't like me back. And that's fine. But I just straight up said "I like you" I didn't mince any words and it helped me move on quickly.
But in this current case, since he probably likes someone else it would probably just make the friendship a little awkward if you confessed.
Leave a love letter. I think trying is better than not doing anything. If it does not work, it will just be forgotten after 100 years where we are all dead. Take your chances while theres still time.
How do others gather the courage to confess thier feelings to the people they're interested on?
Ever since i started realizing i like someone i tend to bottle it up and admire them from the sidelines like some mob admiring one of the main characters. Just like this morning my [secret] crush for nearly 2 years asked me for advice saying he has a girl he wants to confess he said "because you're a girl maybe you know what they wanted or something" and there's this idiot me giving him advice give him detailed advice with a smile while bleeding inside.
What do you think? Do i just have to watch or slip a confession letter to his bag? [It's hard being an extreme introvert ]