I think it depends on the kiss, and the circumstances surrounding it.

Back when I was 18 one of my friends called out as I was leaving a get together, and when I turned around she rushed over and gave me a quick kiss. She had taken numerous liberties in the past, like putting her around around my shoulder when we were walking, or putting her hand on my leg when we're watching a movie because she knew it annoyed me. But that was the first and only time she kissed me.

I wasn't bothered by it, but it did seem pretty random. That said, I probably would have been bothered had she not already made it clear she didn't observe physical boundaries. It was just her being her.

If you're wanting it to lead somewhere though than I don't think it would be a good idea to suddenly kiss him. If he's shy, chances are he tries to avoid confrontations. If he avoids confrontations, he won't really know how to react if you kiss him out of the blue and it wasn't something he enjoyed or wanted. He'd be more likely to engage in passiveness or avoidance, and you could wind up losing him as a friend. If you do kiss him maybe give him some warning first. Like ask him if you can try something, then if he says okay, that's when you do it, then ask what he thought about it and if he wants to try again (meaning right then and there). That gives him a way to prepare for something, doesn't make him feel helpless, and lets you know right away if he was okay with it.