If your snake went from a healthy weight to "really skinny" in only a few months, you may have issues beyond just a hunger strike. BPs can go many months without eating and only lose minimal weight. You might consider taking the snake to the vet. If you can get a fresh fecal sample, getting a fecal done to test for parasites would be good, but of course those are usually not available when a snake isn't eating.

BPs need to feel secure before they will eat. I suspect at least part of the issue is that this guy feels his new terrarium setup is too open compared to the tub he was used to. As Angelica (Daybreaker) suggested, cluttering up the enclosure and blacking out 3 sides (or even all 4 sides in the short term) may help your snake feel more secure. "Clutter" can be things like artificial foliage, or even just crumpled up newspaper. Also, you mention temps & humidity, but not hides. If the snake does not have hides that he fits in nice and snuggly, please add those immediately.