That's a really difficult question, because depending on how he feels, any guys reaction could be wildly different. Even if the feeling was completely mutual, I know that I'd still be stunned, startled, shocked, however you want to say it, but I'm also not your typical guy. So to be honest, the only advice I could give would be to do it very gently/easily, but that could just frustrate him to no end if he's the wrong type of guy.
So, go with your gut feeling and what you know of him. Or, ask him how he would like to know that a girl loves him. And then, use his answer to tell him!
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I am Jon Stewart with some Colbert cynicism, Thomas Edison's curiousity, wrapped around a hardcore gamer sprinkled very liberally with Deadpool, and finished off with an almost Poison Ivy-esque love/hate relationship with humanity flourish.