my boy had the same prob and we went to our PD and gotten probiotics too. but my PD told us its common for babies to poop only once a week. (he told us his own baby's best record was 20days 😅)
i started to give my boy more tummy rubs (morning, after shower, and before slp). it help alot and now my boy at 3.5mths, he poops once a day.
Mummy J