Hmm. Its possible you are reading to far into it. Does he look like he's a little angry when he does it? Is he making it obvious that he wants you to see that face? From what I can see, this is probably just his 'play-face'. He's probably trying to make it look like he's angry just to play with you mind. Do you ever stop and look at him and say "What?"? Does he ever like stop and just start smiling or laughing after doing it? Next time he does it, pretend he is just being a goof and laugh at him. See what his response is.
Now if he hasn't ever stopped and just smiled, I might think that he wants you to know he is thinking about something weird when he sees you. But you said that he likes you back, so he probably doesn't have any other face to play with you mind.
Just laugh it off next time you see him, or better yet, make your own 'disgusted' face and see what he does!