It's true that it's more likely to be adult (breedable) BPs that fast in winter, but all snakes are aware of shorter day lengths (& sometimes cooler temperatures)
and that's another influence. Keep in mind that a snake's instincts are the result of natural selection...and snakes that eat when the weather turns too cold to
digest can die from food rotting in their G.I. tract during the winter (& therefore they do not live to reproduce). So be patient, I know it's frustrating. You might
try increasing the light in the room (or in his cage)...make sure his photo-period is equal to that in summer. Many days this time of year are gloomy with clouds,
besides just having a shorter day.
Another thing: best to offer food when a BP is IN their hide, & just peeking out. Also, offer at night. A snake that's cruising their cage will OFTEN refuse food as
they feel too exposed & vulnerable (to their own predators). Maybe you got away with ignoring these "rules" in summer but now it matters.

BPs are ambush-
predators...they don't ordinarily chase down their prey, but grab it as it passes by them in their hiding place. When a snake pauses to constrict and eat, another
predator can nab THEM while they're preoccupied, so instinctively they're better off if they're not out in the open.
Have you been warming up his prey right before offering? If not, break out that blow-dryer!

If it cools off too much when you first offer, re-heat & retry.