My son is 3.25 years old. We started toilet training a week ago and he has done well with the potty part, but he’s having issues with pooping on his potty chair. He did it just fine a couple times, then had a day where he was constipated and he said the poop “hurt his butt” and he has had an aversion to pooping since. We do put him in a diaper at night time, and he poops within minutes of having his diaper put on. How can I encourage him to poop in his potty chair rather than in his diaper?