try to avoid foods that’ll constipate him like bananas etc just to let his system get working you can try massages to poo or if not i know doctors gave us lactulose but that wasn’t down to food that was exclusively formula
Honestly my baby boy was the same so with his meals he would have a fruit dessert. Usually something like homemade apple purée or pear purée and we noticed even though these fruits don’t have the most fibre, it was the fibre he needed.
Prune purée or prune juice mixed with a little water helped so much with my little one
Our baby was the same initially, but once she pooped she was like a machine and now she poops more than ever. I ended up doing lots of tummy massages to help initially and ended up giving her some lactulose, the poo came after that.