I'm not trying to come off as a meanie here but I may by accident. Snakes are in no way like a dog or cat. With animals that are domesticated, they can understand that you don't mean harm when you are approaching the animal. Snakes don't have that instinct. They know that they aren't in a great mood and there is a thing coming toward them and they want it to leave. By going in and trying to handle your snake during the hissing, you are probably stressing it out more, essentially doing the opposite of what your intentions were. My advise would be to leave him be for a few days and let him shed in peace. Snakes don't tend to like being handled. Even tame snakes dont like it. Its just something they put up with. Snakes are really only concerned with food and finding a heat source. Bodies are warm, therfor a human makes a good place to hang out for a while. I've found that with cranky snakes, best time to handle them is a day or 2 after a meal. They are full, they've been sitting on heat trying to digest, they are generally more content. When they start to get hungry then they parade around the enclosure looking for food. This wouldnt be a good time for handling since they are likely to strike anything that moves. Good luck with your boa. I'm sure he's just a bit upset with his old outfit.