Place a clean finger, nail side down, into his mouth. He will start to suck on it, you will feel his tongue 'cup' around your fingertip. If after a little while you start to feel the tongue then pulling back, and his lower gum come into contact with your finger, he probably has posterior tongue tie. They can stick their tongue out enough to feed, but it takes a lot of work to keep their tongue out like that, so as they tire, it slips back and they are relying on suction alone (not the tongue 'milking') to get the milk out. Plus they are pretty much biting your nipple!