M.I.
breastfed babies will go through periods like this when they grow because breastmilk is so perfect, they use every bit of it up in their bodies growing! :) breastmilk is awesome. so just keep doing what you are doing! :)
its highly unlikely that it is constipation, but you would know; baby would be very agitated.
if you feel like it pump those legs in play or something. after a diaper change or whatnot.
otherwise, it sounds like everything is fine, and as long as he is still eating and sleeping and everything else is ok, just enjoy this "vacation" from poop. LOL