What's the ventilation like? Are you using a tank and if so what lid are you using, or a plastic tub? I have noticed the Sterilite tubs need more air holes than I anticipated they would to avoid a build up of moisture on the inside of the tank. Also, with temperature swings occurring now that it's fall (going from like 68 at night in here to ~85 during the day) condensation was building up inside all the snake's tanks (except the Kenyan sand boa). I added some more ventilation and that did the trick. They stay warm without getting a trip to the sauna

I would put him in something else, take the substrate out and bake it in the oven. That should dry it out. Or just switch to appropriate wood bedding or newspaper or paper towels. (I really hate that coco fiber stuff just in general, I barely even use it for my tarantulas since it takes FOREVER to dry out and gets moldy so dang easy).