Worried about Cats mouth

Question
my cat has a pain in his mouth. it is on the left side of his mouth. we dont know if it is his gums or his teeth. his teeth are not that pretty. he is 12 or 13. when you touch his mouth he cries out in pain and wont let us open his mouth. im worried if it is a rotten tooth or something in the gums. please help.

Answer
At 13 years old I am sure he is suffering with a bad tooth or some seriously bad periodontal disease which is very painful.
He needs to be seen by a vet that can give him a mild sedative and see what is wrong. He may need a tooth pulled or he may need some antibiotics to clear up an infection in his gums and teeth.
Either way, he is in pain and needs a doctor fast!
There is nothing I can do but urge you to get him in to the vet's first thing in the morning.
Please don't wait any longer than that.