I fear that's something out of his control after a certain point: Bad genetics. Oh, those genes did him several favors in other areas, but the guy is about as balanced and coordinated as a newborn giraffe. You should hear some of the stories about how physically pathetic he was back in high school and early on in college. It's a miracle he's come as far as he already has in that department. I fear he's hit his ceiling in this department. He's better off working on his decision making so that he can minimize the mistakes caused by his clumsiness, I think. Work with it instead of trying (and failing) to fix it.
Someone was saying it can help to keep ones balance if one initiates contact in the post rather than letting the defender come and whack into you. Maybe he needs to try more of that, for starters. Maybe that will help keep his upper body upright and allow him to finish more. Otherwise, just working strictly within the framework his body allows will go a ways in correcting his play in the long run, I think.