Most children only move their bowels once or twice a day and many do not move them that often. The signs of an imminent movement are quite clear to a watching adult, and the interval between the child first feeling the need and actually moving the bowels is quite long. If the toddler is emotionally ready to cooperate in using that potty, it is easy for you to help. If you know that he is likely to move his bowels immediately after breakfast or on waking from a nap, delay putting on diapers, plastic pants, jeans or any other encumbrances, so that the whole matter is quick and easy. If he does not seem to care either way or seems to like the idea, produce the potty, stay while he performs and be calmly congratulatory.
Toddlers are extremely contra-suggestive.
The clearer you make it that you really want him to sit there, the less likely he is to want to. Since toilet training can only succeed through his voluntary cooperation, battles will mean certain failure. Tone your reaction right down . Using a potty instead of diapers is just a new skill which he is learning. Feces in the potty deserve a quiet word about how grown up he is getting.