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Provided your husbandry is correct, the first thing I will suggest is that you stop trying to feed him in a separate tub. It just causes them more stress, and that's the last thing you need when trying to get a BP to eat.

Second, I suggest you get a scale so you can track his weight. BPs can go for quite some time between meals with no real ill effect, as long as they aren't losing a significant amount of weight.

The third thing I would suggest is that you try feeding him live. A live feeding doesn't mean you won't be able to go back to F/T later. Sometimes they just need a live feeding as a jump start. Especially since you really have no way of knowing what this snake may have been through prior to coming under your care.