Chewing tabacco and sour in husband's mouth? Cancer?

Hi, My husband quit smoking 2 years ago and picked up dipping. So he just traded one thing for another. He tried quiting the dip but couldn't do it, he freaked out and was miserable if he didn't have it. Anyway, today he said he had a canker sore in his mouth I looked at it, but it didn't look like one, it is a big whole in his mouth. What would it look like if it was cancer?