mouth and throat sores

My father had his first chemo treatment and almost immediately developed mouth and throat sores which led to a domino effect. In one week he lost ten pounds, became dehydrated and had to be hospitalized.  My father also needed two pints of blood.  The doctor is trying a novacaine gel to numb his throat so that he can eat and drink, and some medication to help heal the sores.  Does anyone else have this problem?  We were told in the hospital that the next treatment will be worse for the sores.  My dad has stage IV lung cancer.  nothing we tried could get my father to eat and drink he said it hurt to much.