How do i say hi to a guy i only met once>?

[no]

are you friends with his sister on facebook?
if you are, just say you recognized him from her page...then start small talk..."hey, i work with your sister! small world, right?"
if not, see if you have any mutual friends and do the same thing.
if you don't have any mutual friends just say you came across his page..he won't ask "oh how, you were stalking me?" he probably won't think twice about it.
just say "hey, i'm not sure if you remember me or not, but i met you a few months ago...just wanted to say hey and see how you're doing"

simple. don't be a chicken! what have you got to loose?!