Since it has been a month, I'm more inclined to believe that this is illness related. Usually, not shedding is directly related to humidity but not always.
Is everything else ok? Is he eating well and drinking sufficient quantities of water? Active? Not shedding could be a sign of illness; if a snake is sick and dehydrated, it will not produce the necessary moisture to lift off the top later of skin. I'm sure you've noticed the flood of fluid that appears under the shed skin, right?
I would give the snake a warm water soak...or put him in a moist pillow case or plastic box with moist paper towels (depending on the size of the snake) and see if that helps.
However, the first thing I would do is check for dehydration. Give the skin a gentle pinch behind the head and if it stays in the pinched position, your snake needs to go to the vet as soon as possible.
Dehydration could be a symptom of something else or the cause of death, as I'm sure you know.