In the first 2-3 months babies should generally poo several times a day. It's only after that, as their digestive system matures a bit more, that they start to go longer. So your baby may just be a little behind the curve, as it were.
My DS only started to slow down at around 4 months, he had a few weeks were he went once per week just before we started him on solids, then he was back to every day or two.