Scared of pooping

My toddler has been potty trained for pooping at least since 17 months and we never had any issue or accidents. He's always been great at it. He's 3 now and over the past few months he's had some stuff going on which causes him to be constipated a few times and when he pooped it hurt. Now when he has to poop he's so scared that he holds it and screams in absolutely agony. Like top of his lungs blood curdling screaming. It's honestly so heartbreaking and makes me so anxious. He's 9n a daily laxative which is what the Dr already suggested bc she said it will basically force him to have to poop bit he's still holding it and sometimes the laxative makes his belly hurt and when he does poop it's runny and it gives him a rash so now he's ever more scared to poop. I've never had to dead with this I literally hug and hold him on the toilet as he screams and asks to get down or screams that he's scared. Books won't distract him music won't distract him. And if I don't put him on the potty he runs around my house whimpering or screaming and flapping his hands because he has to go so bad and holding it in hurts.

I'm at a total loss and it's so overwhelming. Tonight he finally pooped but it was on and off all day of this screaming routine. Helppp