I have a friend who seems to adopt new hobbies on a weekly basis. As in, "I would really like to get in to knife throwing, let me buy a knife for that," and then abandons the idea a week later. There is no particular theme to these hobbies: it can be anything from card counting to hatchet throwing. Words like "indecisive" don't seem to fit exactly, because he is very decisive once he sets his mind on something. He is not incapable of making a decision; he is incapable of sticking to a decision.
Is there a word to describe someone like this?