This is COMMON.
Do NOT force it.
Constipation... can be a real real problem... because if the child withholds going to poop, it causes GREAT pain, internally. And then it becomes an emotional problem and fear. And it then becomes a vicious cycle.
Just let him poop in a diaper. He is not ready to poop on a toilet.
And, if poop is withheld, it can cause bulging bowels or encopresis.
Yes, just keep letting him go in a diaper. He is scared to poop in a toilet. MANY kids get this way... and pooping in a toilet, comfortably takes time.
We had to see a Pediatric Gastroenterologist, when my daughter had that problem. He said MANY kids go through that... and it can take months to resolve the constipation and associated pain that accompanies it. If need be, ask your Doctor what you can give him, to soften his poop. But do NOT use laxatives or suppositories. The Specialist we saw, does not recommend it because it is habit forming and is only meant for SHORT term use. A couple of times at most.
Feed him fiber, natural apple juice, pear juice, prune juice etc. These help the constipation to soften as well.
talk with him... gently. He is scared to poop because it hurts and he is constipated. Tell him you understand...
all the best,
Susan