Not pooping 😬 Help!

My son is was born on Sept. 30th 4 weeks early. Up until now he had been exclusively breastfed. Currently we are doing a very slow transition to formula so we can use up all my frozen milk and stretch that out as long as possible. He normally poops every 2-3 days but as of today we are going on a week without pooping! We have tried bicycle legs, tummy massages, juice (per doctors suggestion), warm baths, rectal temp to stimulate him to poop, and mommys bliss constapation ease. Nothing worked so far. Any other tricks to try!? We will be taking him in tomorrow if he doesn't poop but I want to try everything I can to avoid all the sickness at the doctors office!