If I remember right, you son is about 2-3 months old now right? At this time, his smile is still govern a lot by reflex. At least he would smile showing the comfort after the feeding. That's a good thing. After 3 months old, then that's when babies learn smile = happy. 
Massages help a lot as simulations to the muscle. Like draw a smiley face on his face (place your finger on dimple place), or use objects/toys to brush his mouth and surrounding area for simulations. Relax and enjoy the interaction.  |