my son hasnt pooped in a week

so , my son is 16 weeks today , and i switched to partial formula for when i work my 2 or 3 shifts a week (part time retail).. i dont pump enough , but when im with my son he nurses on demand.. but since hes had partial formula , he hasnt pooped since last thursday !!! doc didnt seem too worried about it. he doesnt fuss and it does not seem to bother him, what can i do ???!!!