Why does he make me feel this way?

The reason you feel like this is because he's someone you are either very attracted to or who you like and respect very much as a person (or maybe both :P). Also, if you're still in the closet it could be that the fact he's out is attracting you to him, I know I was pretty fascinated by pretty much every openly gay guy I met when I was still hiding my uality. It's completely normal to feel like how you do, nearly everybody has at least one person whose opinion they really care about. If you want to stop freezing up around him, the best thing to do is to get to know him a bit better so he seems more accessible.