What kind of school is your oldest as OP?Would a change of school help do you think?Or would a tutor and home schooling be something he might except?
I know it's really hard we have 5DC and 2 of our DC are asd and a third is awaiting the same diagnosis.
We also went through the not using the toilet,our DC was nearly 11 by the time we stopped having to use pull ups(well the adult equivalent)and the one day they started going on the toilet and from that day they woke up in the night if they needed the toilet as well.It can be bloody soul destroying when the proffesionals can only suggest the things that you've already one.
We were so worried we even ended up going privately and took our DC to have hypnotherapy.We must have spent a good few grand over the years with different potty's,toilet seats,toilet steps,toilet supports,different mats for the bathroom,outside help,star charts,prizes and stickers you name it we did it and then some and I've worked with children that are asd and none of that helped.
With our DC and the toilet it was because of fear(they'd had problems with they stomach and constant constipation despite a good diet and medication and the fact of not wanting to lose things that came from they're body,having they're nails or hair cut was the same.