This is where that "everyone is different" really comes into play. I NEVER got any mouth sores from the radiation, although by the last quarter, my mouth was sensitive and my lips felt like they had a mild sunburn. However, chemo was awful to my body. Extreme nausea with every treatment and I had to be hospitalized the second time to get fluids back in me.
Again, everyone is different, so maybe he won't have many side effects.