flatter hands. it helps a lot. just tell him to play with his pinky fairly straight. a sort of fish-flop technique. just flop from octave to octave in a 'flutter.' people think you have to be all stiff for octaves. i have witnessed some pretty good octave technique by being sort of 1/2 and 1/2. obviously you want the octaves to stay in place - but you also want a really relaxed wrist because you are repeating them so often.
also, he could try using the fourth finger on the black notes - which sorta helps to balance the useage of fingers. probably already does, right?
i vaguely remember somewhere back - a teacher on this forum suggesting that the inner three fingers be mentally moved more towards the thumb instead of tending toward the fifth finger. he/she said it helped with balance of the hand. and yet you see different pianists do it differently. some prefer to let the fingers side with the fingers and the thumb be on the other side. maybe it depends, also, on the passage and where it is leading?