What exactly is the problem with letting them get to this at their own speed? I mean, a problem that doesn't have distended bowels, bowel leakage, anal-retentiveness and constipation in the future... unlike having a war about pooing does.
Diapers are (still) for keeping the furniture clean. They're to stop having to clean 3 pair of shoes every day. They are for 'until they learn to use the toilet'...
No one learns anything in one shot (or if they appear to, it's because they didn't expose their initial learning to people they don't trust). People of all ages explore new things and gain mastery of them in predictable, circular ways.
Initial attempts (at anything) will be more successful than anyone would expect, then there is a period of backing off, failing, finding a bunch of different mistakes to make, or just dropping it for a while and letting the learning settle into the brain more effectively, a period of time DURING which, repeated insistence that they 'do what they know how to' actively interferes with their ability to learn, and even to do it as poorly as they had been previously. If this period of fallow is not respected, the learning can be permanently distorted, and if it turns out the 'parroting understanding' is much more important in the child's world than actual learning, the child's development will stop right there.
(This is why so many adults can make noises that sound like compassion, but those words and sounds barely scratch the surface of actually understanding someone else's feeling, much less empathizing with them genuinely, because they were forced or coerced into pretending they could be compassionate when they were 4yo and still 8 or 10 years away from the brain development that makes it genuinely possible. They learned the hard way that they were going to have to pretend to be compassionate long before they had the capacity, and once the capacity came by, they'd have no way to explore it or re-learn it, because they were still required to play along with 'already knowing how.')
Either he is 'allowed' to learn the way his brain is wired to learn, or he is 'required' to pretend to learn the way Western culture thinks people learn (in a straight line, on or off, by choice and at the whim of the person 'in charge').
There is a reason 'potty training' is cited in psychotherapy just so frequently... early distortions of learning create big distortions in life, and potty training in particular (being all about the secret body parts and unspoken body products and functions) gets tangled up in power, control, sexuality and deviance.
Work hard to keep control and power struggles out of potty training. It's not your body, and you will never have control over his sphincter muscles.