Hi Mam. Your  baby's  weight is very much low. Babies gain around 30 gm per day in the initial 3 months. So the weight should have been around 4.5 kgs. Proper breast feeding technique is a must to avoid pain while feeding.  He must be sucking at the nipple which causes this problem. Nipple does not play much role in feeding. While latching at the breast most of the areola should be inside his mouth. Can use nipple shield temporarily until pain subsides.