Cori,
As Vin has said, there are several different modifications to the radical neck dissection. The good news is that he should be able to use his arm fairly well just not play tennis for a while. If the accessory nerve is involved then he will have some trouble raising his arm sideways. With time and therapy even that will not be a serious problem.

Mark, 21 Year survivor, SCC right tonsil, 3 nodes positive, one with extra-capsular spread. I never asked what stage (would have scared me anyway) Right side tonsillectomy, radical neck dissection right side, maximum radiation to both sides, no chemo, no PEG, age 40 when diagnosed.