If he doesn't shed properly, small pieces of dead skin can be left encircling and constricting blood supply to the spike underneath, potentially causing the flesh to die.
adequate humidity and adequate hydration should be sufficient but sometimes this can still happen. Try putting a damp cloth on the area and after awhile see if the dead skin can be loosened - this assumes you have a docile chameleon.
If you ever have a problem with shed
coming off go to the drug store and get mineral oil and rub some on your fingers and rub it to the area that shed is stuck.
Maybe its shower time? Lots of peeps use the shower method to get a good misting session in. Don't get me wrong, you don't spray it right on him. Put him on a jungle gym or plant and point the shower head at the wall.