Is he breast fed or formula fed? My son is breastfed and he didn't poo for 4 days. It's normal. His poo isn't hard either and it's still soft. dont't worry!!
To help to poo, my grandma has a unique method which she did to me as a baby. Cut a small piece of soap and insert it into his backside. It sounds like a horror story when my mum told me this... I personally dont't advise. Haha!! Im just sharing ancient ways of doing and not suggesting anyone to do it.