In a novel I'm writing, I have a minor character who is either autistic, or has an intellectual disability. (I know they're different. But like I said, he's a minor character, so I haven't really decided yet.) Either way, my setting is fantasy, mostly inspired by 9th century middle east, so both conditions would be bundled up as "Simple".

The character never gets a POV, but we see his actions, he is a part of his family, we see others interacting with him. The story demands that he be far from being able to function normally (otherwise, he would have been next in line to the throne). While the character is a very minor one, it is important to me to avoid clichés, and to write him as a character rather than an object that other characters relate to. I don't want a "noble savage", and I don't want someone very disruptive. I do want someone who feels real, neither exaggerated, nor underplayed.

How do I do this? I've tried to look for information on the web, but most of what I find is either very general ("all autistic people are different"), focuses on the family without giving any information about the child and his abilities, or is followed by vast amounts of criticism for being imprecise. So I don't even know what's true and what's not, what's common among people with autism and what is rare-but-photogenic.

An additional detail: the plot spans about 50 years, so while this character would not appear often, he would make appearances as a child, a teen and an adult.

Would it be reasonable to make him have autism, and use a letter board for communication?

How would a person with autism respond to a situation with a crowd, colours and noise, such as a royal wedding?

What should I avoid by all means if I don't want to offend, but to give a positive representation?