Half emptied poop

So we caught Joshua pooping outside of the bathroom and rushed him into the bathroom. He resisted to sit but we finally had him sit on the toilet and a small bit of poop came out. I told him to sit more but he resisted and wanted to pull up his pants and diaper. He was wearing a diaper because it was his bedtime already. One minute later he starts bearing down in another room. I rush him to the bathroom but he already pooped in his diaper. What does this mean?
He still feels uncomfortable to sit on the toilet with underwear for 3 minutes. We were almost there.