This is gonna sound kind of gross but my son had this problem the first year of his life so we used glycerin suppositories to help him poop, one every couple of days to help him poop. You could also try apple juice, or cranberry juice.
Idk maybe ask him and see what he says. He could be traumatized and that’s why he won’t
A couple hours after I posted this, he went outside for exercise with his Grandpa and he fell down the slide on his face. He got to the bottom, stood up with a bloody lip and said, "I pooped!" So now he's scuffed up, but still pooping.
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I would just take him to the ER now.