Rather typical snake behavior- it takes patience & some will just do better than others at staying calm while handled.
Try to remember why BPs feel safest in their hides, & try to "be his hide" when he's out & being handled. In other words, his fear is instinctive when he's out in the open- so many snakes feel safer when cuddled next to us (as if we're a big rock to hide behind), & you might also try handling him in a pillow case or under a towel, to limit his vision & wandering, until he learns to feel safer with you.
I don't think his being an adult now will make any difference, by the way. Just be patient & try to empathize with where he's coming from.
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