There’s a sentence in this that every single reply to this has either ignored or missed, and that’s the part where you think he’s autistic.
From the small snapshot of his life and personality that you’ve offered it does seem that he shows some pretty clear signs. It may be that he doesn’t even realise. I know that I’ve very recently come to realise that I’m obviously autistic and I’m very much an adult. How everyone around me throughout my entire life missed it/didn’t realise is absolutely boggling.
Whether he’s diagnosed or not shouldn’t change that it should be handled with the appropriate sensitivities and equality policies as if he was autistic. But that’s entirely up to your work place and it’s culture.
You all need to remember that while you ‘only’ have to be around then during the times you’re around him, he has always got to deal with being autistic, whether he knows he is or not. And from the sounds of things he may not be very good at masking, which is both good and bad for him. As a person who seems to be neurotypical, you live in a world that is designed for neurotypical people. He isn’t and doesn’t. Imagine being forced to live in a world where you need wheelchair ramps, but there are none provided - anywhere. He needs mental ramps.
You are more than entitled and allowed to not want to deal with him or be around him, please don’t take this as saying that expect you to do that. But there needs to be sensitivity and an understanding of his struggles. If he is autistic, he cannot help the way he approaches situations or how he feels when you rebuff him. To him being told he’s annoying is clearly something he’s taking very, very personally. Take it from someone who is also autistic, it’s horrible. I feel like my entire existence is being rejected, and it sticks and I ruminate on it for hours sometimes days.
So speak to HR first, see what their equality policy is, and what options that they have. Hopefully the company culture and policy recognises that a diagnosis isn’t always possible or needed. And take it from there. Ultimately I think that some of the responses about finding time where you can separate yourself from him is the most likely solution.