Well, my guess is that his sucking reflex is going away. Happens around 3 to 4 months. So with the sucking reflux they will turn and "suck" on anything, a finger, etc... and the suck is more reflexive and involuntary when the reflex is observed. Then at 3 to 4 months it is normal for that sucking reflex to disappear, and then volitional sucking takes it's place, meaning the baby learns how to use his muscles to make himself suck in a voluntary way. Most babies make this transition without difficulty, some babies have a bit of a harder time. Is he still taking in the same amount, or has that changed? I would suggest trying a nipple that is wider at the base, so that it fills more of his mouth (avent for example) and see how that goes. HTH