Your instincts are spot on.
This is nothing to do with your ND. There are many weird people who are bullies and "solo woman" is a prime target for them.
I'd say the impact your ND has is that you're thinking there's something wrong with YOU when he is the weirdo here.
- Grey rock. Can you have a sick episode, how long are you actually there for? Have you got allies or colleagues you can sit with?
- Intense with intense.
You know when you go to a shop and there's an annoying person who talks shit?
That's now your new personality.
Ask who his mums name is and where he went to school and then ask what his GCSEs are and what his favourite colour is. Then ask more questions, if he preferred maths or English .... Then start asking "what's 25 times 4".
I assume you're classy and professional in your mannerisms...bring out your inner weirdo.
Approach him first and start talking loudly about his Fun Crazy Outfit even if it's a suit. Start using phrases like "you're so MENTAL" in a high pitched voice.
If you see him first zoom in on him and insist on taking a selfie with him.
Don't give him a moment of peace. Don't let him control the vibe.
Get loads of memes on your phone (photos of kittens and babies and Christian quotes) and insist on showing them to him.
If you have his phone number send him a photo of a kitten in a basket every few minutes. Don't respond to anything he says, just keep sending the kitten.
Start talking about your relatives and make up long, boring, stupid stories.
Read something really stupid and inane online and then have a monologue on how interesting it is ready to talk to him.
If he tries to change topic of conversation then smile and switch straight back to Whether Or Not Cardi B is hot or not.
Give him a nickname and use it again and again. Make up a rhyming phrase with it in.
Keep saying "you're just like my new uncle buncle wuncle" to him. Be WEIRD.
A lot of people with awful pushy manners love polite people as they think they are submissive.
So go opposite. Change the dynamic. If he says about work go "beep beep bonk me is bad" at him. If you see he's ordered macaroni for dinner start bombarding him with questions about the macaroni.
You're not criticising him or actually being aggressive so there's nothing HR can do.
Other colleagues really won't give a shit if it's not affecting them. Act normal with them.
You want him avoiding and being scared of YOU by the end of the stay.