My brother has autism, and like many people have pointed out before me, it's important to remember that autism is a spectrum. One of the most glaring misconceptions that people have about autism is that it appears in only one form and can only be portrayed in one way. So, like those before me have advised, first determine where on the spectrum you want your character to sit on as it will heavily steer the direction of your story.
Secondly, the number one thing you don't want to promote is making it seem like autism is somehow a disease that needs to be cured. Don't demonize autism. Certainly, you can include that as part of your character's conflict, but don't ever make it so that the character's autism is somehow magically 'cured.' Symptoms of autism may change over time
(autism may become less severe over time) and autistic people may learn to pick up social cues, but autism cannot be cured. I can say that, at least in the case of my brother, he's gotten better at communicating and working with his autism, but such a thing may or may not be the case for others. Just remember that being different isn't inherently a bad thing. That is probably the most important thing to keep in mind whilst writing.
(Also, don't make an autistic character into some cute pet of another character. I've seen some questionable novels that make an autistic person seem like some cute little baby animal the main character takes care of, and it honestly makes me feel a bit uncomfortable. Though, this is my personal opinion and not necessarily my brother's.)
Thirdly, I thought that I'd give you some tips when it comes to writing autism based on my own interactions with my brother. You should know, however, that my brother is pretty high functioning. Sometimes, I even forget that he has autism, though that may just be because we're very close. One of the first things that people notice with my brother is how he doesn't make eye contact, and secondly, how he often pauses for a very long time before answering questions. Simple questions like, "How was your day?" or "Are you hungry?" will just be returned with pure silence. Since my brother doesn't say a single "Hmmm," or "Let me think about that," or give a small tilt of the head to indicate he's thinking, many people end up thinking he didn't hear them or is ignoring them. It's a small quirk of his, but it can get on people's nerves when they don't understand that he has autism. It certainly makes waiters feel awkward when he's ordering food lol. My brother also spaces out often and will often not respond in a middle of a conversation. I'm not sure if his mind wandering away is a result of his autism, though, or if it's just one of his personal quirks. Routine is also incredibly important to my brother, and when something unexpected happens, regardless if it is good or bad, it can deeply upset him. A surprise trip to Disneyland was not appreciated by him when he was younger as it interrupted his plans to make a new level on Super Mario Maker. Besides that, I know that my brother has a far stronger emotional attachment to objects than other people. I still remember him crying when my parents sold an old van of ours when we were kids, and to this day he's still very particular with his possessions. There's a lot of other quirks I could share, but I'm sure my brother would kill me if he knew I shared anything more.
Finally, I just wanted to share some words of encouragement. I can tell that you take writing autism rather seriously, and I know that dealing with such a delicate subject is nerve-wracking. But, as long as you approach the topic with genuine care and tact, I'm sure you'll do semi-well. And honestly, for someone without autism, that's the best you can hope to do. Also, my brother says that you shouldn't be too scared when writing since autistic people are very, very diverse. There's no perfect way of writing autism just as there is no perfect way to write a neurotypical person. I think it's great you're willing to tackle the task of writing the alienation someone with autism can feel. The desire to be normal is already something someone without autism struggles deeply with, it's hard to fully comprehend the pain someone with autism can feel.
Anyways, if you've made it this far through this post, I can already tell you're a determined soul. So, I'll leave one more resource for you. There's a book written by Naoki Higashida, a boy with autism, called
The Reason I Jump that may be very helpful to you. It's formatted like a Q&A and provides a lot of insight into how a person with autism interacts with the world. Well, best of luck!
