I think you just have to keep forcing that big open mouth before he latches. With James, I always had to open up my mouth wide to encourage him to do the same before latching -- for some reason, he was more difficult than the girls, he would keep sliding back into bad latching habits if I didn't force the good latch. I also found that making sure my nipple was aimed towards the roof of his mouth really helped as well, it seemed to get us latched right. Have you got some lanolin to help with the pain? And like Katy said, has he hit that "distracted" phase yet where he goes to look at something and turns without letting go? Ouch!