Every baby is different. Keep smiling at him when you change him and feed him. Talk to him, looking him in the eyes and smile. He'll catch on. My granddaughter was a frowner. She hardly ever smiled, but by a year she got the idea and became a smiler.
My daughter didn't smile til she was 3 months also. And at 2 months she had that pouted out lip when she cried (it was SOOO cute!) He's still a newborn! Absolutely perfect and he will be smiling, crawling and cooing before you know it... then shortly after walking, talking and peeing everywhere! (at least that's what happened with my daughter...)