There's a great deal more
to aikido than just ukemi. Does he have a problem being uke for pins as well as throws? those aren't all that painful on the back unless he can't bend over at all. Maybe for throws he could be Uke to the real new guys who need to move slowly, who aren't going to toss him hard and who could benifit by working with someone with experience.
Human tendency is to get upset, frustrated and give up. A more mature personality will make adjustments and do what is necessary, working around the problem. He doesn't want to wind up unable to walk at all right?