E.R.
I'm on my 2nd breast fed baby and as long as the baby is eating basically as always and peeing regularly I wouldn't stress it. Especially if when he does poop it looks pretty much like it has all along. Breastmilk pretty much guarantees he won't have constipation issues. - Caveat, if he's getting an iron or other vitamin supplement that might cause some constipation, especially iron.