My baby stopped eating since he developed mouth sores, how should I treat them?

Offer fluids to him. An older infant's mouth sores are usually caused by viral infections. The gums, cheeks, palate, and throat may have some pain, so the infant eats less. Parents can offer fluids frequently, and hope their baby drinks a little bit each time (just enough to keep hydrated for a few days while the infection resolves on its own). If a baby won't drink enough, then he needs I.V. Fluids in a hospital.
Orajel, . Any of the Benzocaine containing products will help numb the mouth so the baby can feed.
Tylenol, (acetaminophen) bland foods. Sores in the mouth are usually caused by a virus and are not serious. However, they do hurt and can cause your child not to eat. Infant Acetaminophen (tylenol), bland foods, and "magic mouthwash" can aslo make eating more comfortable. Magic mouthwash is a mixture of maalox and benadryl (diphenhydramine). Check with your pediatrician for the right dose for your child.