we were in the same boat - the day olivia woudl poop we wouidl get excited because we knew the next 24 hours she would eat and sleep well, then it would start again - gradually over the next 2-4 days she would struggle and fail to poop, sleep worse and get upset - real constipation - wish we had added water or changed formulas earlier, but it did get better eventually.
besides the poops being hard - the key sign of constipation is straining and struggling to poo with nothing coming out. it is normal for a little one to strain sometimes (even get red in the face) and if they do so and get a result and it is soft than that is totally fine. it is when they keep struggling like you describe that it is constipation and not "learning to push". if he is suffreing it is not "normal" in the sense that you don't have to just accept it - i would continue to offer some warm water and try doing "bicycle legs" to stimulate him, and put him on his tummy for a bit as this can help too, i rub lo belly in circles and when she would try hard Dh woudl help her along by pushing her knees in towards chest and giving her help to strain against him / bear down :oops: whatever it took (we seriously used to cheer when there was poop it made such a difference for all of us) good luck.