The humidity actually wasn't all that low. Staying above 50% is fine, but closer to 60% is better.
4 months is nothing in adult BP world. It can be frustrating, but it's not uncommon.
We can definitely help you out. But please, PLEASE start learning from a reputable source.
Wherever you heard a mouse can basically come back to life and eat it's way out of the snakes stomach...well, don't learn anything else from that source. That's honestly one of the most ridiculous things I've ever heard.
Yes, live mice can carry parasites and disease, but if you buy from a reputable feeder source you won't have a problem. You're probably thinking wild mice. You definitely don't want to feed wild mice you catch in the yard or whatever, that's where disease and parasites come from.
If he's not losing considerable weight he's going to be fine. BPs can go up to a year without food if healthy.
After looking at your enclosure picture....
First, you could use more clutter. BPs crave security and that enclosure offers very little.
Unfortunately, those stick-on dial style thermometers and hygrometers are wildly inaccurate and basically junk. They can be off by as much as 15% or 15°.
Also, with a few very small tweaks you can return the fogger/mister. You can make a few little changes that will help your humidity control drastically.
I'm not being rude here, just trying to help you and your pet. It sounds to me like you would benefit from doing more research on your pet. The more we know about our pets the better we can care for them.