My best guess here since it looks like he's well kept, well fed, not ill and so on is this is just an adjustment thing. This is an adult snake, who was in the same home for his (or her has it been properly sexed) home for it's entire life. Even if this snake is still in the same enclosure your house is a completely foreign environment. You are a completely unknown quantity still.

I've seen it called LONHS syndrome (lights are on but nobody's home) by the Barkers in their book, another place it was referred to as Maladjustment syndrome but basically they were both saying that adult snakes aren't mindless. They do suffer from a stress disorder when their lives are changed overnight. Sometimes that's expressed by refusing to eat or by being nippy or a whole host of other behaviours.

I've seen it myself with various rescues we've had and adults we've purchased. At first they act one way, then a month later another way and so forth until they finally settle back in to their basic behaviours. I'd just say give it time, stick to simple routines in all you do with the snake, that helps them understand that when you do A, then B will result. Don't let his antics drive you away, glove up if you need to but just go about your business of caring and feeding him.

Also some snakes just hiss a lot but don't much strike. I've a couple of adults here, that always hiss when you enter their space. That's it though, big hiss and grumble act and nothing else. I think that's just their way and they like to complain a bit LOL.

Of course, if you suspect there is any illness at play here, seek the advice of a qualified and experienced herp vet.