You can massage his tummy and bicycle kick his legs, that tends to help sometimes if he still doesn't go I would call the pediatrician and ask what the next step would be. I know for older babies a tiny bit of apple or prune juice does the trick but I wouldn't give it to a baby that young without asking the ped

call your pediatrician I would my daughter got constipated a lot when she was a baby but I always hesitate to give out that kind of advice because every baby is different I think for her mainly it was her infant supplement that I was giving her they tend to be heavy on the iron which can constipate the baby's