I have three children who all suffered from constipation as infants. They were all breast fed. Everyones milk supply is different. I have a really small amount of milk that I make. I was never able to produce more than 2 oz at a time. The babies just had more frequent feedings. I think this is why mine went through constipation, or so I thought. A breastfeeding infant can actually have between 12 bm's a day or 1 every five days and still be normal. I believe the more milk you produce the more bm's the baby has and vice versa. My youngest is 8 months and now she is finally having one very large bm a day, but that only started once baby food was introduced. I wouldn't be too worried unless your baby is straining to have a bowel movement and it is unproductive.
My now 9 year old did the same thing for 3 WEEKS we had to have a visiting nurse come out 2 times a day, the suppository wasn't helping she would stimulate him The doctor ordered him to have a little caro syrup it help him go, Well that made him too fussy, too much sugar. then out of the blue one day he went, only to be followed by massive diarreah. I would bring him to see his doctor. they might be able to help
Good Luck
Michelle