Toilet troubles!

Hello

so my 4 year old has just got a diagnosis of ASD. Along the way he has decided that the toilet is the absolute devil and won’t sit on it. 
he has started holding bowel movements which makes him irritated and grumpy. 
I have tried; toilet books, adding colour to the water to make it more fun, bribery (not ashamed to admit it), me pretending I don’t know how to sit on the toilet and getting in all sorts of weird positions, and literally nothing is working. 

He would much rather hold it in or go in his underwear. 
I always go for the soft parenting touch but he will literally hit the roof everytime I try and get him near the toilet. 
He started off potty training amazing and took a while to sit on the toilet but he has recently just started to point blank refuse to go for a poo. 
the paediatrician suggested it could be because it’s something he understands and can control but this is a habit that isn’t healthy. 

thank you every one! 

Phill