Maybe you need abit more lead up... talk to him for a few weeks about the dimishing nappy supply perhaps. Let him see how many nappies are left and that they are running out- letting him know he will have to wear undies then. That way he has some preparation and understanding.

Perhaps try having a puppet tell a story about how he was scared to go to the toilet but then he tried it and it was okay. It sounds like he has some fears/anxiety about it for sure and trying to gethim to share exactly what that fear is mayhelp. Remember that fears are rarely rational so don't expect something logical and brush it off as nonsense if he reveals it. My friend's son is scared to do poos in the toilet because he thinks it will be huge and go all overhis bum... at least she knows that and can talk about it with him even though that is what happens in a nappy!

Is he still at kinder? Did his teachers comment on any developmental concerns they noticed? Do you feel concerned about his behaviour? Given he can physically hold on he clearly doesn't have an issue there but there could be a barrier he is facing that is making this really hard for him and finding that out first may help.

All the best!!!