A lot of children hate doing poo on potty, because it's so gross in their little opinions.
At this age, you can talk to them about the fact that the toilet is best for taking away the waste. Spend time flushing, singing, sharing books about toilets and so on.
Some people just leave off the nappy until the child has no option, but that can lead to constipation and emotional stress for many kids.
Hopefully you are talking to him about when you need to go, how good the toilet is at getting rid of stinky poo, how much better you feel afterwards etc etc
It can be a real nuisance for him at nursery, because staff will be fine with dealing with wet pullup pants, but won't want to clean a dirty bum unless he has other special needs.
He will eventually get the idea that poo goes in the loo, but I agree it's a bit grim until he dies.