One of my cats has become very nervous over the last comple

How big is the cut on mouth? Did we come back in like this from the outdoors?

Are we eating and drinking normally? Are you able to clean the cut on the mouth?

Are we having any urinary or fecal accidents since the tail is not working so well, or can we lift the tail enough to go to the bathroom?

Does he feel warm to the touch? Over his tail base do you notice if there is any swelling or tender area? Or in the back half of the back region?

I am thinking he may have gotten into a fight with another cat, cats have a very back bacteria in their mouth called Pastuerella. When they bite, the teeth inject this bacteria into the tissue, then the skin closes back up. Then over the next days a deepening infection starts to grow under the skin. This may take many days or even a week to show up. However when it does form an abscess the area can become very painful and sore. They can start to act much like this. I would recommend to have him seen at a clinic. They will likely need to get him on antibiotics and anti-inflammatories before this condition gets any worse.