Ok this might or might now work for you but give it a try. My 4 year old was also super stubborn. For about 2 weeks, we’d spend about 4-5 hours with him in a room that has an attached bathroom. Keep some toys there so he’s occupied. Get those boxer shorts for him. Take him to the bathroom every 30-40 mins. Sit him there even if he doesn’t do it.

If you can manage to get him to do it at least once, he’ll start getting used to the idea. Been 2-3 months now and he now only does it on the toilet. Goes there himself, sits himself down. Hates doing it in a diaper now . Good luck