Short question: What do you do to help your toddler who has issues with going poop, and who screams when he thinks he might poop, and during pooping? I'm pretty sure they are psychological issues, not actual constipation.

Long story: He is 23 months old, and has issues with pooping since before 8 months old (when he was seen for his first "urgent" visit for bloody stool due to holding it too long). We have tried all kinds of fibrous foods and even constipation medication but the issue isn't hard poops... it's that he holds it on purpose. When his diet is more fiber filled, he takes a day or two to adjust and begins holding it again (4-7 days is the norm). He has been going on the potty occasionally since 8 months old so he can let some monster poops out (all these monsters are over a foot long and scare me). We are now starting potty training and he is equally resistant to pooping on he potty, in underwear, or in a diaper. Is there an age at which this gets easier? Is there something I can do to make him not hate pooping and not scream in anger every time he thinks he might poop?