M.R.
C.,
If he is in Occupational therapy I would speak to his therapist about body awareness. My kids had difficulty understanding what thier bodies were telling them, and at 17 and 10 they still do not feel the urge to go as often as they should. Both potty trained late, but we mananged to get it done by age 4 for both of them, but it did not really happen until after 3.5, and it was a very labor intensive task, but you can do it. He may not have night time control for many years, so don't stress about that no matter how much other people suggest that it is a problem, just buy good nights and take it in stride.
The only real advice I have for you is a general rule that will be helpful with everything you teach your son, since his language development will always be iffy, you should try to put everything you tell him into an active statement instead of telling him what to stop doing. Like, "Put the pee and poop in the potty" instead of "don't poop and pee in your pants." Tell him what you want him to do, and avoid making statements that he has to reverse (he has trouble doing this-especially since autistics already have trouble distingusihing prepasitions.) Any statement with over, under, to, for, etc. will confuse him, so be sure it is as simple and active as possible.
I understand the frustration, but it will make you feel better just to know that you are not doing anything wrong, many of us have struggled with this issue, and the temptation to judge ourselves as mothers based on what is typical for kids off the spectrum is common. You are doing a fine job and it is a very frustrating struggle.
good luck,
M.