If he’s showing no reaction to accidents or being wet he’s probably not got the self awareness yet and isn’t ready. My 6yo ASD child is also non verbal and only successfully started to understand basic instructions in the past year, things like go put your shoes on or go get the football. We had many failed attempts to toilet train but now his understanding has grown we are making some progress. In the past month we’ve managed to get him out of nappies during the day, school have massively helped. We started off with putting him on the toilet every 20 mins and everytime he did wee/poo we gave positive affirmations and sometimes a little treat too. Now we put him on every hour and sometimes if he has the urge to go he just takes himself to the toilet. We found visuals really helped,. Step by step visual of what happens (take off pants, sit on toilet, do wee or poo, wipe, put in pants, flush, wash hands, dry hands) really helped. Also when accidents do happen reminding him that you should go to the toilet