A bowel movement with every feeding is common in the early weeks. However around 6 weeks of age some babies change their stooling pattern from very frequently to once a day, or once every few days, to even once every week or so. Your exclusively breastfed baby is not constipated as long as the stool is still loose (pudding like) and a mustardy yellow colour when he does have a bowel movement. The stools should also be substantial.
All About Diapers (what goes in, must come out)
There are natural laxatives in colostrum which help babies expel the meconium. The more breastfeeding your baby does in the early days the more colostrum he takes in and the faster the meconium clears. As the milk volume increases, he will start to have transitional stools.