I also don’t have much advice, but like the above says - ask for a sit down with nursery staff and ask them for advice. Explain you have been trying, what do they suggest? Also, my sisters son was the same - he would hold his wee all day if we were out and then release when home. She just kept trying him in public toilets when out and about and he’s got much better now
Not much of advice but you could try with a portable potty? If he gets comfortable he can bring it to nursery and then see from that how it evolves? Don't know if it's a crappy idea because then he will still have to change eventually to a normal toilet 🤔 any help from the health visitor?
Also there are several groups in this app, parent support for children with autism, maybe join one and ask them if any had had the same situation. Probably many. And how they overcome it. Good luck!
I don't have any experience, but I would ask for a sit down with the nursery manager and his key worker and explain the situation