Firstly I would take Charlie to the vets to ensure there are no underlying problems. I suggest you try 2 litter trays 1 covered and 1 without a hood, You could also try and put a few drops of rescue remedy in his water in case he is little stressed. Perhaps something is frightening him and he does not want to leave his bed.
When you want to change from one type of litter to another it needs to be done very gradually, each time you change the tray put a little more of the litter you indend to use and a little less of the old.
If all else fails and a Vet has conformed there are no underlying problems, we have found that if we put a cat in a large dog pen with 2, 3 or even 4 trays, and just a bit of bedding, (Obviously waster in a hanging pot) cats will retrain very, very quickly and use the tray once again. I hate the idea but sometimes it needs to be done and you cannot live with the cat not using the tray.