Just not his usual self

I feel very sad for Charlie and is very afraid that he may start chewing on his skin to relieve the irritation. Actually, when I saw your thread earlier, I was going to suggest you look for parasites because they usually attack around the rump. The colony nest mostly in the creases on the cage, any cracks or openings on perches and covers. During the light hours they would be in their hiding places and would only come out in the dark to feed.

To check for mites, you should do so during the night after it has been dark for a while. 1) Listen for scratching noises or foot stamping by Charlie which in most cases is a telltale sign that he is being bothered by mites. 2) Use a flashlight and carefully examine perches, cage wire, covers, surrounding areas and Charlie. 3) This is a very good method also. Completely cover cage with a white bedsheet before you take lights off and then return a few hours later to examine the inner side of the sheet. They are very tiny and looks like black specks of dust or very red if they had fed already and can be seen scurrying about to get out of the light.

I have heard of a parasite and a type of minute worm that bores through the skin and inbeds itself just beneath the surface. The infected area is said to look like a reddish pimple I have never personally seen them so I can't vouch for it.

If Charlie can't restrain himself from biting at the affected area and you are not able to see the vet until after the next couple of days then I suggest that you put a collar on his neck which would require you to towel him. You can use the cardboard cylinder on which paper towels are rolled on to or light plastic tubing about the same diameter but I reccommend the cardboard tubing because it is lightweight and much safer. Slit an opening down one side, measure and cut an appropriate length to put on his neck and then glue, tape or staple edges of slit together so that it can't be removed. The proper length to cut would be long enough so that it prevents him from bending his neck towards his body but yet allow him the freedom to eat, drink or clamor about without discomfort. At first he may find it very annoying so you must have enough time to spend with him until he settles down. If for any other reason, it would provide a distraction.