http://www.tmz.com/2018/02/07/donald-trump-hair-blowing-air-force-one/

Donald Trump had a surgery to remove his bald spot in the back of his head (called Scalp Reduction) a procedure that has been discredited largely by my efforts because it produced significant scarring in the scalp and the bald area that was removed almost always returned. In addition, from the video, I can see that the hair in the back of the head is very, very thin suggesting that he has a depleted the donor area (possibly from hair transplants which is just a guess on my part). It is hard to know if he is wearing a partial hair system without seeing him up-close and examining him., but considering the amount of hair that he has, a good hair system with his unusual styling, could produce the cover he has.  He was, unfortunately, taken in by some of the terrible surgeries offered in the 1980s and early 1990s.