My LO struggled with pooing for the first few weeks. She’d go sometimes 5 days without going. We didn’t know that it’s more about the consistency rather than frequency so as long as they’re not in pain, poo isn’t hard when comes out they should get the hang of it. My LO now goes everyday sometimes twice. If they’re worried take them to the GP and see if they have a milk protein intolerance
Formula fed babies don't poop as much as breast fed babies but if your little one is struggling then use a premade for a day or two as the premades act like a laxative, so they will relieve some discomfort for baby and should flush them out