My 4 yr old has a canker sore on the inside of this lip, every time he eats he cries in pain, is there anything otc we could try to help?

Magic mouthwash: is a prescription medication that contains viscous lidocaine, diphenhydramine and magnesium hydroxide ( or similar). Two of these ingredients, such as Childrens Benedryl a Maalox) are over the counter and can be combined in equal parts and applied with a cotton swab to provide some relief. A dose of acetaminophen or ibuprofen along with the topical preparation can also be used. Best wishes