Not pooping

My boy is 7 weeks and I'm breastfeeding, but supplimenting an ounce of formula after the last three feedings of the day. He has never been a good pooper. He will go days without pooping. Dr has said to give half a child's suppository butbdoesnt want him to become dependant on it to go. It's been 5 days since last poop which was achieved by suppository. We have tried bicycling legs, tummy rub, warm bath, and stimulating with rectal thermometer. Anybody else going through this or know what could be the problem? The next step is tummy xray and I don't want to go there...