I had the same concerns about my son, but when I spoke to my pediatrician, she had no concerns.
Below is a Q&A which may assuage your fears:
Q. I have some concerns for my 3 month old grandson. He only has bowel movements every 5 or 6 days. They are not "hard" or difficult for him, but just doesn't seem normal to me. It's almost like he has a "blow-out" on those days and doesn't do it again for the same length of time. Just doesn't seem normal for a baby that age because he's a good eater and seems to have no problem with the urine. Dr. has told her to give him prune juice, but he's on breast milk, and of course hasn't started anything else yet. Is this o.k.? Thank you for your help. Buka, TN
A. For an older infant or child, going five or six days without a bowel movement would usually be a problem. In younger infants, especially those who are breastfeeding, constipation is defined more by what the bowel movements are like rather than how often they occur. So a younger infant would be constipated if his bowel movements were like hard, little pellets, or if they were very large, firm and difficult to pass. Some people also consider an infant to have constipation if the bowel movements have a consistency that are thicker than peanut butter and the child strains to pass them.
Keep in mind that babies who are exclusively breastfed very rarely get constipated. After having very frequent bowel movements during the first month or two, they then begin to go much less often. In fact, some breastfed babies only have bowel movements every week or two. In these children, as long as the bowel movement is watery or soft when they finally have it, then they are likely normal and not constipated. So it sounds like your grandson doesn't need any treatment, since it doesn't sound like he is constipated. In cases like this, I personally don't recommend giving an infant prune juice or doing anything else. Giving extra water, apple juice, or prune juice would be appropriate treatments for an infant that really was constipated though.
Why do breastfed babies have such infrequent bowel movements? Most people believe it is because breast milk gets digested so well that there is not much left over to make bowel movements. Of course, once you start feeding him solid foods, like cereal, that will likely change. At that point, he will likely have more regular bowel movements and they will probably be firmer.