I honestly dont know how to make him stop, but I can relate. My son pulled hair between the time he was 1 and 2 and he was in daycare. The teachers would keep the hair b/c sometimes he would pull a big clump of hair out of childrens heads. It was like a vice grip, once he got a hold of it, the teachers would literally have to pry his hand off the childs hair. I dont know why he did it, but eventually he did stop. We just used time out and made sure to encourage 'nice touches'
We've been encouraging our daughter to pat nicely. Its helps that she has the cat to practice on. Our cat is to fat and lazy to bother her if she pulls her fur.