What do you do when the guy you like thinks you are stuck up?

First think on it objectively. Are you stuck up? If you think you might be a little, you probably are. Then decide if this is something you want to change about yourself. Maybe it is, maybe it isn't.

If you don't think you are, ask a couple of your more straight forward friends, and do not get mad if the answer was not what you were looking for.

If it does not seem you are stuck up, maybe it is a misconception he has. You can maybe ask him why he thinks that about you. If he is honest with you, maybe you can let him know why it wasn't true, or you can take what he says and think it over. He may or may not have some valid points.