Help! My son cant poop!

So my son is 3 months (14 weeks tomorrow) and he still can’t poop on his own. We have to give him karo syrup to get him to poop. He can pass gas fine without it. He used to poop normal until about a month old. He was born 37+5 so they considered him premie and he is in the 10%. Does your baby poop on his/her own? Is this normal or something I should be worried about? I thought surely he would be able to poop on his own by now. He have tried many different kinds of formula and nothing has helped.