Toilet training our son

Morning,

I am new here and just looking for some advice from others who are in our situation and may understand. 

Our little boy is turning 6 shortly, and we are at our whits end trying to initiate potty training.

He has sensory processing issues alongside his autism, so trying to navigate a wet or dirty nappy and break the cycle/introduce toileting on a potty or toilet is becoming increasingly difficult.

Has anyone who has been in a similar position got any advice or tips on how to introduce potty training successfully?

I appreciate this is a time thing and won’t happen overnight, but we have attempted to leave his nappy off for longer periods of time, but he will just hold onto this and ultimately have an accident.

We have introduced a removable toilet seat and have gradually sat him on this to show him this is where he is to toilet, and have done nappy changes in the toilet for a while now. None of this seems to have a positive impact so we are struggling.

As I say, any advice or tips from anyone who has been in a similar position would be really appreciated.

Thanks in advance x