My baby is 11 months now but I remember being concerned about her not pooping for a few days when her poop was changing types into more normal consistancy. I can't remember how many days it was. I would call the Dr. though especially if it is any more than 3 days because I have heard of a baby having blocked bowels and the baby had a really hard time and it did long term damage to the baby and the parents did not know it was so serious at the time.
Try rubbing his anus with a warm washcloth. I have tried this when she is constipated and sometimes it gets things going.
35 year old 2nd time mom