2 week old not pooping

my son is now 17 days old fully breastfed. up until now he has been eating and pooping normally, a dirty diaper after almost every feeding. thursday (15 days old) he had fewer poops than normal. yesterday he only had 1 poop in the evening. today he hasnt pooped once. my milk supply has been good, i wake up full whenever i get more than 90 minutes of sleep. however he tends to fall asleep after only a few minutes sometimes he will stay latched for up to 10 minutes but once lets go i usually cant get him to take the other breast. i feed him whenever he acts hungry though. so i am unsure if he is eating enough or if he is constipated. i an going to start feeding him with a bottle so i can see how much he is eating but i would like some advice. if he still hasnt pooped normally by monday morning im making a doctor appt to have him weighed. he had been gaining weight really well before. was up almost a whole pound from birth weight at his last appt when he was 12 days old.