Let’s assume it’s possible for you to see him almost regularly, perhaps because you’re both in the same school or university or workplace.
Let’s consider the alternatives. You can be just subtle but he will hardly notice you (don’t worry, he won’t think you’re a freak). You can also let him know about you, but you may not get what you want in the end: just walking up to him saying “Hi I am **** and I like you” is a way of doing this.
Actually, you can be both subtle and get to know each other over time.
I suggest you to approach and talk to him. Am I serious? Yup, but since you’re asking this question chances are that also want to know exactly how is this even possible. There a number of ways but the easiest is to use a gimmick.
An example of a gimmick can be a flyer (of a festival, exhibition, etc.). So while you’re handing out a flyer to him, you can casually ask “So what do you think of this event? Will you come? Why not? What kind of events do you prefer? Oh, really, how’s it like? Sounds cool to me too.” As a bonus, you can strike conversation with other boys too: it doesn’t hurt to expand your choices.
If you don’t have such gimmick, it can also be made out of thin air. Have a pen and paper and approach him to ask his help for your survey (from your teacher or your parents or your boss or your friend). You can ask his opinion about topics such as video games, movies, music, arts, or pretty much anything. If you run out of questions then just browse Quora. 😉
Your goal isn’t to tell him you like him but to get something going so you can begin to know each other. So, also let him know of your opinions, what’s your experience on the things he does/doesn’t like. It may or may not end with exchanging mobile number or befriend in Facebook (it’s a plus), but that doesn’t matter much at this point.
Ok so that’s for the first time. While you can do similarly for second time, you don’t have to! You can just say “Hey, you’re that guy from last time!” or maybe both of you already knew each other’s names. And of course you can smile. Then you can either start a conversation or go on with you were doing, but at least you and him are no longer strangers.