The thing is that it shouldn't be the case that you are 'forced' to send him to ss (if you don't want to) just because this particular school is spectacularly useless and unwilling to provide adequate support for him. This school is failing him, is failing to meet his needs not because, from the sound of it, he cannot access the curriculum there, or cannot participate socially, but because they are not providing adequate support for him.
If you want him to remain in mainstream school, this is entirely possible if he has the right kind of support. LAs and ms schools generally are lazy, shortsighted, blinkered and ignorant about what kind of support is possible through expert teaching. For so many children with ASD, teaching requires a different approach to motivate. But it's not that children cannot be motivated, it is just a different approach is required, one which takes a bit more work, and will cost more than a TA who is paid minimum wage and could be just any old person from down the road, with no training. It's insane that the people who are given the most challenging job are paid least of most of the staff in a school.
Like pp, we did ABA which continued as support in mainstream school. We had to go to tribunal of course to get it, like most, as LAs are reluctant (because it challenges their thinking, as above, and because it can cost them more in the short term).
Sorry for rant, you can tell it is a particular issue of mine 