The issue is...ABA is designed entirely to get the child to mask heavily - the whole point is to make them appear "normal", which is why so many have problems with it.
It was designed in a time when "appearing normal" was the absolute priority, when deviating even slightly from typical meant you'd become an absolute outcast from society. However, in today's world (outside the US, at least), awareness is growing to the point where society is far more tolerant of those who appear different even now, so putting that much effort into "appearing normal" isn't actually beneficial any more (and certainly not worth the cost).
As things stand, from what you've said, it seems like the only barrier to your son's social progression is the toilet problem (with the best will in the world, nobody's ever going to get anywhere socially with wet trousers...I don't think society will - or should - go that far...), so maybe drop ABA from the thought process, and focus exclusively on that? Like I said, this is not an unusual problem even for autistic adults.
For my part, if it helps...I'm very rules-based, and always was as a child, so I had a set process for going to the toilet (order of operations), and a set trigger/schedule for it (the end of a class, or the end of a TV programme if at home). It helps that my mother's also autistic, so she knew exactly what she was dealing with and trained it into me from a very young age; to this day, I still use exactly the same triggers and exactly the same process for going to the toilet (same for baths, getting dressed, going to bed...).