E.,
This sounds like my son. The pediatrician says he's just hording all those nutrients and there's absolutely nothing wrong with him. He's now a year old and tipping the scales at 24 pounds. And perfectly healthy.
When I was most concerned was about the same age your boy is at now. He would go five days without pooping. But when he did, it was a huge blowout! Messy, brownie mix disgusting! I counted my blessings I only had to deal with it once about every four days.
And once again, my pediatrician said - and says - he's perfectly healthy and growing exactly as he should be. Even now, it's just one poop a day - but usually every other day. That's just the way he's made.
If your pediatrician says he's good to go and his poops are neither a runny watery mess OR hard acorns, I'd say listen to her. Your little guy is probably just perfect in every way!
Good luck to you,
J.