Seems clear to me HR keeping you in the dark is intentionally trying to cause a problem. Hanlon's Razor cannot apply when it's specifically their job to manage this. They are probably expecting you to abuse him and get rid of him? You accidentally became aware and ruined their plans. While the target is most likely on his back, you have a target on your forehead if it goes sideways.

By you not being made aware, you get the 'oh I didn't know' excuse. But in my experience middle managers are who take the fall. Some CxO will be able to come in and be the 'I would never treat my employees so poorly, pack your shit middle manager and get out'

Lets also be realistic, you're a manager of at least 1 developer and this is the first time you're dealing with autism? Impossible. It would seem to me you have a hiring practice which is excluding autistic people? Quite a significant portion of developers are autistic. Let me guess, he's practically the only person who accomplishes anything on your team? No insult to others, but I bet he's a 10x engineer. He produces so much that you can't ever lose him? He's your brent from pheonix project?

Being sued, or otherwise legality is difficult to say as it depends where you are. So I won't go there. This is also not a matter of 'both sides', he's the one with a disability and you probably legally must accommodate? You wouldn't say someone in a wheelchair has to change to handle the stairs. But the problem is so dire that you can't ask him for his needs. So this relationship has already failed?

I have bad news for you as well, autism and relationships, yikes. It's probably 99% on you to fix this. this post is a fantastic start, always ask for help when you don't know what to do. You definitely came to the right place if you get what I mean.

You also don't state what is making you crazy. Probably the wrong word? Avoiding the C word most likely is step 1.

What are his challenges? Autism is a spectrum, not everyone has the same challenges.

Personally I find the question, 'what accommodations' a curious one, especially work from home. Before I self-realized autism, my boss seems to have accommodated me completely. My boss is amazing, I suspect a guy who just spent 4 months to buy the perfect fountain pen, might at least understand me if you get my drift...

So what is the accommodation that I want exactly? There's nothing I seem to want.

I suppose this needs to be a chapter in my book, but lets try to figure out some generic catch all accommodations?

Pre-steps, make him aware you are aware of autism and that you will be making changes to make his life easier.

1. Eliminate his need to communicate and social. Make everything entirely optional to him. Get him a virtual personal assistant? I've never had one of those, but perhaps they can translate what he needs. I don't get it personally but I'm not a team player. Not because I can't do it, but because it always goes bad 100% of the time. People HATE you for literally no reason. You are experiencing that right now with him.

2. https://en.wikipedia.org/wiki/Pathological_demand_avoidance PDA is something I suffer from. If someone micromanages me, controls what I am doing, or otherwise makes a demand from me? Lol not going to work out. I have no obligations placed on me, but that means I either do it to myself or I get to be productive. When solarwinds had their boondoggle, I decided to code an open source project from scratch to entire replace and eventually be better then what solarwinds ever had. I am incapable of doing nothing, so it's not like I become lazy or something.

3. Sensory issues are already resolved by being a remote worker. That's on him to figure out. The only lightbulbs I can handle is 2700k. For the longest time I had 5000k and I wasn't aware how much psychic damage it was doing to me.

4. routines and no deviation or change. I'm not sure how this applies to you. If you suddenly message him and want a meeting right now and he's doing something. That's not going to work out. If you have a standard meeting window which may not be used. Perhaps you only work with that personal assistant?

5. Some huge puzzle/project for him and only him to work on. Imagine, what could the business use that might currently be too much budget but you think is not an achievable task. If you give him this puzzle to solve. He will deliver the impossible in like a month.

Hopefully these are helpful.