Not good! You should get him to a vet for an X-ray to determine if he did indeed eat the battery. There is corrosive material inside batteries that could cause a lot of tissue damage inside if the battery starts to leak or the battery itself could cause an obstruction. Sometimes you can look in their mouth & see where there are mouth sores from the battery- there could be sores on the tongue, inside of cheeks.  Your dog should see a vet ASAP to ensure he is safe from the acid & an obstruction.