Open Question: Our almost 3 year old can say he’s hungry, thirsty, wants more, ect, but can’t say when he has to poop! why?

Our son gets offended when he’s called a baby, or some one tells him he’s messy or bad, but don’t mind when he’s called stinky or is told he stinks. He knows what toys are his and which ones are his brothers. When we can’t find something of his and haven’t seen it in days he knows where he had it last. He says when he’s thirsty, hungry, wants more, when he’s hot, cold or something hurts. He brings his cup to you and says it’s empty and wants more. He’s a smart kid but he can’t just grasp telling us when he has to poop. What can we do? We’ve tried to make it fun, we’ve tried making him sit on the potty for a period of time after he dirties his pants, we’ve scolded him when he uses his pants and we’ve cheered when he has used the potty. He doesn’t even once in a while use the potty and go back and forth. He always goes in his pants. We have been fighting with him for a whole year. Our other children had it down in the matter of months. What can we do?