With my boas the hissing has always been a defensive reaction, so it sounds like it's that over a feeding response. Also when my boas grip really hard is when they are feeling a little insecure: some of mine that I've had for awhile never really wrap around things anymore when handling (they'll just kinda be there and let me support them). I would drop the temps, leave him alone for ~5 days with his tank covered, then try again and see how he responds. When did you bring him home/how many days ago?