After the first few days of life, it's fine for a breastfed baby to go 7 to 10 days between poops. This doesn't mean he's constipated — unless his stools are hard. Some babies poop several times a day; some don't dirty many diapers at all.
If baby is growing (in weight, length, and head circumference) and not showing signs of sickness, there's no reason to worry. Just enjoy the lack of stinky diaper changes. (And be ready for that one HUGE one every so often.) When you begin to introduce solids, baby will probably begin to poop more regularly.
If baby usually poops multiple times a day and suddenly skips a week (or vice versa), go ahead and call the doc. Either pattern is fine, but a big change in bowel patterns might be cause for concern.