Hissing doesn't mean aggressive by any means. I find my aggressive snake do not hiss at all. Usually they are already in the striking position the second they smell me or anyone else who happens to come near their rack. My friendly snakes hiss when I startle them though. So this may be the case for you. Going on a hunger strike, especially if it is colder then usual, is normal. I wouldn't worry about it yet and if he isn't losing any noticeable weight then it is a good sign that he is just not hungry.