My stepdaughter did the very same thing when she was about 4. The doctor called it "Tricotealomelia" (sure the spelling is wrong). It is caused by stress (her mom was seeing a new man during this time and it was causing her a lot of confusion and stress -- of course our minds went to the worst possible scenario, but we resolved that quickly. We were at a loss. We talked to her, asked her how she was feeling...finally when the whole left side of her hair was gone (she did it at night, in bed), we finally had to tell her that if she did not stop pulling her hair out we would have to cut it because it was long on one side and bald on the other. She cried and I felt awful, but we told her it might help to tell us how her heart felt instead of twisting her hair into knots and then pulling to get the knots out. After a week of this (so hard for us to see her pain), she finally managed to stop, slowly, and we told her as long as she was trying that was all we needed. Over the course of a month she had stopped. When her hair came back, it came back curly! One side straight and one side curly. It eventually fixed itself, but it was a very difficult time. We took her to see a therapist but he told us she wasn't really responding and to give her a few months. She seemed OK, but to tell you the truth, she has never really been a joyful person. She had to grow up too soon to watch over her little brother and to this day, while about to be married, she still doesn't have that twinkle in her eye. It is a sad situation when parents divorce with very young children. We always thought it was for the best that way, they'd be too young to remember anything. They understand way more than we give them credit for. Good luck with your son.