My toddler is exactly the same!! It’s so frustrating 😩
My LO was the same so stopped for a while, then slowly introduced it again asking he needed a wee and eventually got back on. Now he is just wearing pants during the day and we keep putting him on the toilet every hour.
Mine is scared too, the flushing sounds trigger him and every time I have to let him know when I am about to flush so he is prepared.