I wonder if he has a canker sore on the inside of his cheek. A canker sore is from a type herpes virus, a lot of us carry this kind of herpes virus that cause canker sores and it spreads easily in families from sharing glasses or kissing and such...ask your doctor on Monday about it. It's nothing to worry about and it's not the same kind of herpes as the STD.
I know when I get canker sores in my mouth they itch and are bothered by brushing my teeth and toothpaste. They can take anywhere from a few days to a few weeks to clear up and go away. Sometimes a mixture of baking soda ad water (mixed as a paste) rubbed in it will help, so will salt water.
Hope this helps, and again, it's nothing to worry about...they will go away. Good luck!