To think his behaviour at the zoo was OTT?

Like you said, he's autistic. They have obsessive behaviours and once they start something, they have this deep-seated compulsion to finish it. This was a trigger for your partner. However, he is still an adult and capable of going to a Zoo in the first place. I would have gone and sat down while he finished it by himself.

I think you need to have a firm talk with him and advise while you understand his Neurodiverse nature - He is still an adult capable of making better choices and being mindful.

Endometriosis causes pelvic pain mostly, sometimes back pain. Usually around menstruation but it can occur outside that time.

You were looking for him to understand your illness and make accommodations for it. Are you being as understanding and making accommodations for his disability as you want him to be for you?

My dh is autistic, you don't have to stay with him but if you do you do have to have a level of understanding that he does have a disability. Sometimes that disability will manifest itself in ways you find difficult to understand. He did leave with you, he did give you the 'comfort' you needed. Did you do the same for him?

Honestly,

If I was your OH and knew that you were 'done and ready to go and then you had a convenient flare up I'd be pretty pissy as well.

Endometriosis

I’d have gone back to the hotel on my own and left him to it. Ridiculous behaviour.

I had an Endo flare up

Can someone explain what this is please?

Is it Prague zoo by any chance as that zoo is a pain in the ass for walking. Honestly I’d have gone back on my own and left him to it.

I think yabu cos it's not as tho he didn't leave. He was just disappointed about leaving. As an autistic person I guess he just says what he feels. But he did acknowledge you feeling ill is more important than his need to see all of the zoo. And he did agree to leave.
So nothing has really happened here.
But he's autistic so you might be waiting a looong time if you expect him to automatically not express what he thinks are his logical feelings for fear it may be taken as hurtful by someone.
I'd just let this go tbh.
If he'd refused to leave and forced you to go round the whole zoo that's a different story.
But basically he just got briefly annoyed that things didn't go how he expected them to go.
I think that's probably the best you can expect. I have autistic friends who would be like this. 'High functioning' but yeah they will get annoyed if a plan changes. They had an idea of how to do things then that suddenly has to change. They paid money to do something they now won't be doing. Yeah they'd get upset.
The issue is if they carry on being unreasonable. They can't help initially getting upset about it. That's part and parcel of being autistic.

He has a point.
If I go somewhere to visit something then I want to spend as much time as I can there / see everything there.
That's pretty normal behaviour whether you are ND or not.

Re you feeling unwell, I think that depends.
Would sitting at the hotel be any easier than sitting on a bench or in a cafe whilst he went off to see more? How far from the hotel were you ? How easy would it be for you to go back on your own ?
If I were with my partner at something they were enjoying and wanting to see more of, and I started feeling unwell, I would go and get a drink and find somewhere to sit and rest there.

I would have left for the hotel. He could catch up later.

A grown man pouting because he doesn't get to see caged animals suffering in a zoo, that he's already been looking at for several hours? Not caring that his girlfriend is feeling unwell?

Yeah, unless he's under the age of five he sounds pretty ridiculous.