Ok, none of this is meant as criticism. Just my "thoughts" as to what may be going on..

From what you have described, I feel like this snake had a bit of a roller coaster ride. First kept correctly, then under fed for a while to the point he is underweight, then fixing this, then the humidity and his hydration going waaay down, only for you to raise it way up and actually forcing him to soak, etc.

Going from one extreme to another can cause overall poor health. Not that I'm saying you SHOULDN'T have fixed your mistakes asap, but you see how this up and down could cause less then perfect health. If the snake is actually weak and lethargic at this point, I wouldn't force soaking. Its stress, esp. for a weak snake, and stress can kill. Your husbandry needs to be SPOT ON now. To the point humidity is raised and there is moisture on some moss and substrate. Yet not to the point that everything is soaked completely. Simply the way it should be for a BRB. The temps should be correct. Not higher or lower then normal.

Once everything is on the up and up, just leave him be. There really is nothing else you can do at this point that won't add to the stress. I fear in his condition it wouldn't take much stress to finish him off.

If anything, go to the vet. It is stress, but you will give him his best shot that way. But stop handling him and moving things around. Let him rest. Observe, but hands off as much as possible.

In my humble opinion only of course...

I used to have a Brazilian Rainbow Boa from Jeff Clark and visited his facility a few times. Wonderful snakes. I hope yours pulls through. If he does, try to keep the care and husbandry perfect. If he doesn't gain weight, you may have to feed more often. Not overfeed by any means, as that will start problems as well, just bump up a couple of days. However, until THIS issue is resolved completely, do not attempt to feed. You invite re-gurge and that would probably be the beginning of the end for this snake in his condition.