Your pediatrician needs to get with the times! As a fellow pediatrician, the latest research is definitely against starting cereals as the first foods, especially before 6 months. (There is good evidence suggesting that this may be a significant contributing factor to obesity in infants and children, even if they are breastfed.) I always recommend starting an infant on pureed fruits or veggies, or starches like sweet potatoes, and skipping the cereal entirely.
Don't stress about the duration between poos unless your baby is in significant pain or distress, or unless you start noticing that the poo is becoming very hard, (like little pebbles), which is very uncommon in breastfed infants.
One of my colleagues, a peds GI doc, says he has cared for infants who go 10-14 days between big blowouts, and are totally normal.
There are certainly medical conditions which can cause serious constipation in an infant, (such as Hirschsprung's disease or cystic fibrosis), but you would likely have other symptoms or a history that is concerning, (such as delayed passage of meconium or failure to thrive, respectively).
You might consider a natural probiotic supplement if he seems to be in a lot of discomfort. I like Culturelle, (the vegan, hypoallergenic formulation is best, as the regular does contain trace cow's milk protein), as it is probably one of the best researched formulations. I give it to my own little one to help with her gassiness, and it's great. I just break open one capsule, dissolve into some EBM, and give by syringe. Hopefully you're pedi is up on the use of probiotics in infants--one of the latest studies was just in this month's Pediatrics!
