We had no real luck with difficult child 3 before he was 3, with bowels. We ended up using bribes. He was in child care at the time, and the carer did a lot to help him here.
But he was very slow to learn and it turned out to be a sensory delay issue. He simply didn't recognise his body signal. I was still being called to the pre-school and later the school to clean him up. He started school at 5.
So, what worked for us - for bowel training, we stuck a small box of M&Ms to the wall above the toilet. They were for difficult child 3 when he first used the toilet. We never bothered with the potty - we just went straight to the big toilet, with a junior seat. However, we found the junior seat was a problem so we dispensed with that as soon as we could.
It took a while but regularly putting him on the toilet after each meal for five minutes eventually paid off - it was pure chance, but he earned his reward. We stuck up another box.
After he was earning a box a day, we changed the rules - no daily reward, but he would get a BIG box for going a whole week with no dirty pants.
We didn't use shame in any way if he made a mistake - we just showed him how to clean up, helped him with it and began the week over. Too many mistakes making a week out of reach - we went back to a small box for going a day with clean pants. And so on.
You do what you feel you need to. You sound like you're going through similar problems to our difficult child 1 - he was phobic about the potty and the toilet. at three, he would refuse to use t he toilet OR the potty, but he also was trying to keep his nappy clean. He just didn't understand that it had to go SOMEWHERE! It took a screaming match once a week with his grandfather, who was the only person who could "bully the sh*t out of" difficult child 1. Literally. It took about six weeks of this, on a weekly basis, for difficult child 1 to work out that this was going to be a continuing problem and he may as well embrace toilet training and accept the inevitable.
difficult child 3 wasn't phobic, just unaware. When he was seven he was toilet trained during the day, but we had wet nappies every night. We tried getting him up to the toilet as we were going to bed ("come on, honey, it's toilet time") but it was only pure chance if we kept him dry. We finally took him to a specialist in bedwetting in autistic kids. We were organising a pad and bell system but had some exercises to go through first, which involved putting him in pants and being prepared to change the sheets several times a night. We had charts as well.
Then we discovered - putting difficult child 3 in pants had been the trick. We did it in summer which is always the best time for practical and physiological reasons, but the nappies (good disposable ones) had been TOO efficient, they hadn't let him feel that he was wet, so he was not getting the necessary biofeedback (bladder feels full; void bladder; feel wet; uh-oh, should have gone to the toilet). The pants at night did the trick for him.
With difficult child 1's extreme potty phobia - we used bribes again. He wouldn't sit on the potty bare-bottomed even for a bribe, so I bribed him to sit on it fully clothed. His rear end brushed the potty fleetingly and he demanded his reward. OK, he had earned it - just. But I kept upping the ante - he now had to sit on it with a bare behind. Then he had to sit for thirty seconds. Then he had to sit for a minute. Then he had to sit long enough to produce something. He would 'perform' with bladder, but that was it. He learned fast and had dry nappies from then on, but as I said, the bowel training was the worst.
Now I must emphasise - my boys were not being disobedient - they simply couldn't help it. They had poor connection between body signal and physical function. Then it was a matter of understanding and learning to cope with life - not easy, with autism.
A classic example that had us in stitches at the time - difficult child 3 was school-aged. He WAS bladder-trained at night, so he must have been at least seven years old. He got up in the morning and went to the toilet (always hits the door sounding like a SWAT team raid). We heard him voiding a healthy bladderful, and then we heard him exclaim in annoyance, "EVERY MORNING there's wee!"
He'd been hoping that needing to empty his bladder (and everything else) was something he would grow out of and never need to do again. He was waiting for the morning when there would be no urge to go, no need any more ever to use the toilet.
He was most indignant when we laughed, and very despondent when we explained.
So not only do our boys FEEL differently, they also think very differently. It can be frustrating, but it can also make life very lively. And DO make sure you write down any such things because you DO forget, even when you think it's unforgettable.
Oh, and I almost forgot - he need never be too big for nappies. They come in adult sizes, or at least the disposable ones do. And I do know that you can get hold of cloth ones as well, I have a quadriplegic friend who has now lost bladder & bowel control, she'd had some de luxe ones made for her so she can be out all day and only need changing when she comes home at night. She gets help to get washed and dressed, they then help her into the car and she spends the day driving around visiting friends (the car uses hand controls). She's almost illiterate but she's written a book (which I edited and helped publish) and she's now working on a second, if only she would stay home occasionally and get more writing done!
She is a classic example of how if you think outside the square you can have an active, happy and interesting life.