Pooing

My 6 week old has stopped regularly pooing. About 2 weeks ago we traveled to my mums house for a few days, we wanted him to meet my family but the second day being there he didn’t poo and when we got home he still wasn’t pooing. We initially thought it was the travelling but now he will only poo every 4-5 days. I am breastfeeding so I though it might be a problem with my milk or he is not getting enough nutrients. I have seen my GP, we had an x-ray done and all but can’t figure out why he isn’t pooing. In the x-ray you could see he is backed up. We have tried newborn laxatives, purified water and raw sugar but nothing has worked. Has anyone else had this problem with their newborn?