It shouldn't ruin a friendship if he's mature enough about it. If he doesn't like you back, he'll just reject you and you'll have to move on emotionally. Romantic feelings between friends can happen, it's not unnatural. Things will only be ruined if you don't take the rejection the right way.
That's why communication is probably a good idea. If you tell him upfront, like hey I have some feelings for you, not only will it likely take a load off your shoulders, but also you'll be able to see what he thinks about it. It's a little exhausting to play games to try and impress him and get his attention because he likely won't get it since you're just a friend in his eyes. It's also extremely easy to misinterpret behaviour; just because he looks at you a certain way or is listening to you it doesn't mean that he's romantically invested. It often happens that if you like someone you see him in another light, therefore skewing the reality that he probably isn't into you like that (yet?). If y'all don't talk, it'll just be a mess of misinterpretations.