Hi A.,
I have twin boys that are 15 months old and a five year old boy. When my twins were about 2 months, one would poop everyday and the other would not. In addition to being constipated, he was very uncomfortable and spit up a lot more than his twin. Their diets were identical. They were mainly breastfed, but I did have to supplement about one bottle a day with formula. I took him to the chiropractor and began regular chiropractic adjustments. I also began him (and then later his twin brother and the five year old) on probiotics. I started with Bio-K - I put a little on my nipple prior to feeding just once a day. Later, I added a powdered probiotic called HMF powder to his bottle. Both of those things worked like a charm. Once we got him going, he still poops everyday or usually twice a day. He doesn't have to see the chiropractor as regularly any more. However, everyone, including myself takes a probiotic everyday. I'd highly recommend it especially during cold/flu season because it can really help their immunue system. If you think about it, a big part of our immune system is in our digestive tract. Our bodies need to fully absorb the vitamins, minerals, micronutrients, etc. in our food for our immune system to work optimally. Probiotics help the body to digest and absorb that nutrition. Probiotics also eat up the "bad "bacteria in the gut which also boosts the immune system.
Our doctor was not that concerned with his constipation and I hear others say that "breastfed babies don't poop as offten and that's okay." It's really not okay. This bodily function is how the body rids itself of toxins. If you don't poop, the toxins say in the body. Over a long period of time, a person can develop "leaky gut" where food travels through the intestine walls and that is not good. That's an extreme example of what could happen, but better to get things cleaned out and avoid those potential problems.
Good luck!
C.