It's best to pull after they've been worked. When they're warm, the pores open up, making the hairs pull out easier. So take him for a little ride, or lunge him for a little bit, then get to work.
Take your time, and give yourself a couple days to finish it. It's best to spread out sessions on super sensitive horses. He can't feel any pain of the hair being pulled, but what he's not liking is the tug and pull on his neck.
^Lynn's method of pulling is one of my favorites. My gelding is fine with pulling out the mane real fast in the middle of his neck, and base of his withers, but up close to his poll, he's more sensitive, so I'll do it this way, and he's then relaxed and fine with it.