He will get it.
Just not yet.
Pooping is often the last 'stage' in toileting.
It can be an 'anxiety' and a physical thing....
When my daughter was that age, although we did not force her to poop... she got anxiety/stress about it. She then got constipated and started 'withholding' her poop. This then causes PAIN and the poop to get harder. It then gets harder to come out. It then causes pain, to come out. It then causes the child to NOT want to poop. Then a vicious cycle ensues.
We had to take our girl to a Pediatric Gastroenterologist. He said he see's this all the time, everyday. A child he said, should not be pressured to poop. They will get it. On their own. If not, they can get medical problems, like my daughter, and not want to poop... then constipation and/or "Encopresis" can occur... and then, they have to be treated, medically. Meanwhile, it is ALSO an 'emotion based' issue for the child. For my daughter's case... it took about 3 months, to overcome.. and to get her stool to a proper consistency and NOT be constipated.
Withholding poop... is uncomfortable, causes pain, blocks up the bowels, can distend the bowels and other things. Which requires medical help.
So now, with my son... I do not pressure him.
MANY kids, poop... later.
all the best,
Susan