We might not all agree here on this, but if that was my snake, I'd feed him mice to get food into him. The only trouble with mice is that they aren't big enough to be
an adequate meal for an adult BP, and many BPs (but not all) won't eat multiple items per meal. But especially since you think he's losing weight, I'd feed him mice.
Some will tell you to stay the course (rats) & wait for him to be hungry enough to eat them, but I hate to see a snake losing much weight, because at some point it
may compromise their immune system too...& then you really have a problem.
No, don't feed your snake in another cage or container...ever.

on you for stopping that silly practice 2 years ago...it's just all wrong, well-intentioned maybe, but
wrong nonetheless. And I agree, "force-feeding" is a last resort...actually I'd tube-feed a snake long before I'd ever force-feed, as it's way gentler if done right & can
be enough of an energy charge that they feel good enough to thereafter eat normally. But still, try mice for now.
Also: what are the cage temperatures & humidity? & how are you measuring them? These things can make a difference in whether or not a snake will eat. Same
goes for their sense of security (with adequate hides & cover). What about handling? (have you been? if so, how much?) Really have to cover all the bases here.
Almost forgot: are you feeding live or f/t? How are you offering f/t? (temperature? how thawed?) etc.
If live, snakes CAN become afraid of their prey, esp. if they get a nip (even if you don't notice, they may remember), so it's possible
he feels intimidated by rats for this reason...& I don't blame him a bit, rats can really hurt, even kill a snake.
While I certainly advocate feeding snakes on f/t (or freshly pre-killed, "p/k") whenever possible, sometimes feeding a young rodent
that's live will get a BP "going again" (feeding). Rats & mice with eyes closed do NOT bite/fight back at all, & shortly after they open
their eyes, they are still fairly "clueless" (safer for snakes that need to stimulate their instincts again).