When I had that problem with my daughter I would give her a bottle of soy formula. It could be your formula you might want to try a diff kind. Check w your pediatrician.
Are you sure he is constipated. Newborns have weird bowel movements when they are little. They can go 3 times a day, every 2 days or every 4 days. It switches up a lot. But trying massaging below his belly button with two fingers for a few minutes and over his legs in a bicycle motion
I'm going to warn you about the massive blowout hes going to have make sure you have something under him cuz it's going to go everywhere and definitely don't forget to line the car seat because that will get so nasty and is difficult to clean
Baby water with the kayro
Try adding a little less formula to the water.. this helps the baby get a little more water to promote a bowel movement. I had to do this everyday to atleast one bottle so my daughter would be regular. Baby suppositories can be used as a last resort.