Try 1/2 prune juice 1/2 water. That's what our ped told us to do! And it honestly works like a charm. Our boy poops within the hour of prime juice lol. Our son was extremely constipated and did not poop for nearly 6 days and they walked us through how to do a suppository that was over-the-counter.
When my LO use to have trouble I would give him water. If your giving him water, maybe try giving him some between feeds that’s what I do and he usually does it in a couple of hours or by end of day.
My ped told us to give our baby prune juice straight up. Up to 4 oz a day. She wouldn’t take it straight but we put two ounces in 2 of her bottles and it definitely worked
Apple juice! Works like a charm for us
Miralax and water. More water, warm bath and gentle belly rub. It will come don’t do a suppository yet just wait
Blackberries and prune juice
I would call your pediatrician. They’ll prob tell you to bring him in and they may give you a glycerin suppository. My older son had to have those when he was about this age.