Trouble with toileting

Hi.  I have a 5 year old son with an ASD diagnosis and I can't get him to poo on the toilet.  I think I've tried everything, but if there's any advice out there, I'd be really grateful. 

He is currently in mainstream education with a one-to-one teaching assistant.  He is entirely dry, day and night and will NEVER have an accident!  He will happily tell me he needs to poo and ask for a nappy.  He's not afraid of the toilet itself, and is not worried about the noise of the flush.  When he poos in his nappy, I will tip it into the toilet and he'll wave goodbye to it as we flush (delightful!) but as soon as he sits on the seat, he says it's gone away or it's broken.

We have tried bribery - there is a beautiful, wooden rocket which he is fully aware of and would love to play with, but it will remain in it's box until he can achieve the illusive toilet poo.  Nothing is incentive enough, it would seem, to make him do it.

He says that he scared of the splash, so we have put toilet paper into the bowl before he tries, but still he won't let it happen.

Any ideas??

Thank you

Suzie