He may enjoy the reaction he gets from you. Toddlers are notoriously self-centered; he doesn't realize it hurts and what's more, doesn't care. Other moms have posted about this question and solutions on Breastfeeding.com; you might check it out and see if any of the info is useful to you. I also like Dr. John Rosemond's childrearing advice - you can find his columns on line or check out his books at the local book store or library.