I think it sounds like he is preparing to shed. My BP starts to become lethargic and inactive 1-2 weeks before he shows any of the visible signs of an impending shed like clouded eyes. He also stays in his warm hide all throughout the shed process. My theory is that he is saving energy to grow the new skin or maybe at that point the new skin is already growing and it makes moving uncomfortable. Snakes are also vulnerable when shedding so they prefer to stay hidden.
Anyway, unless your BP is limp and unresponsive in his hide I wouldn’t be concerned - most likely he is going to shed and he starts to experience the physical effects before you can see any noticeable signs of it.