Sounds like he likes you
He loves you
Shoulders or waist are the natural places to hold when hugging someone. When he is holding your waist he is giving you a more intimate hug.
Because he is being intimate with you
Could mean he likes you and was testing the waters or it could just mean he thinks of you as a good friend.
He is either uncomfortable so just holds you anywhere or maybe he just thinks you are so insanely awesome that he just has to grab you
He's obviously going to hold your waist or shoulders, where else do you want him to hold you when he hugs you???