What Causes Swelling In My Dad's Cheeks?

hello my dads cheek is all swollen he seems to think its a apsis under his tooth but now he has a big blistery type thing that starts from the back of his mouth to the front.. he wont see a doctor as he never goes out and dont like meetin people.. can u help me by tellin me what it might be and what he can take to ease the pain as hes in alot of pain... thankyou..