Sounds good, op. Don't over think it.

OP here. A lot of helpful possibilities have been mentioned; thank you for posting everyone. Regarding the petit mal theory, yes that thought crossed my mind too. Doctor gave some indicators but so far no evidence.

I guess I'll stop worrying about the seeming incoherent drawings and writings and as a PP said affirm my ongoing interest in listening if he ever wants to explain.

My ASD daughter used to obsessively write notes and times tables. She was often secretive about it, but eventually she opened up and said she was trying to remember things and was worried about forgetting. She has comorbid OCD, but this symptom could be from ASD or OCD.

There is a subtle difference I see between ASD and OCD. ASD makes it hard for her to express her thoughts and feelings in general. But she is aware and can express thoughts and feelings related OCD symptoms, however she may be ashamed of the obsession and so try to hide the reasons for it.

Hope this helps.

This isn't a condition. Art does not have to have meaning.

I would guess that most likely it is a language issue—my DS is super verbal, but has a really hard time connecting the dots of his thoughts for others.

Your description made me wonder though—could he be having petit mail seizures? My father had them and would "wake up" writing mathematical formulas upside down with no memory of doing so. You mention he has ASD, which can co-occur with epilepsy. Not to worry you—just a thought.

He could also be having a lot of anxiety about using language to explain. It's probably really hard for him to calm down his imagination and excitement and to use words instead of the pictures. He's totally thinking in pictures or totally immersed. Totally typical for our non-typical thinkers. Doesn't surprise me at all that he doesn't want to be pressured into sharing/expressing this. I would just let him know that when/if he is ready to share or talk about the drawings, that you are interested and would love to learn about them. My ASD child is like this (not about artwork) but about other things he has big feelings about.

Rather typical of someone whose intelligence far outpaces his ability to express it, OP. He's probably working out a new code/language without even realizing it might be a big deal to you. Autistic children have done incredible things.

Don't push him on that, but work on language and social skills generally, as much as you possibly can. If he's going to contribute to archeology or science, he needs to be able to communicate adequately to find a job

My son was like this as a toddler/preschooler. He's now in middle school, and doing well in a gifted and learning disabled program.

He has an imagination? Stop grilling him and let him explain if and when he wants to?

OP- social pragmatic issues get broken down into expressive/receptive language. In my opinion, it's not that your DS doesn't know what he was trying to draw (even if it's abstract), it's that he has language processing impairments and truly can't verbally explain it.
He probably hides the pictures because it's a tremendous amount of effort to explain what he drew. I would suggest making leading comments about the picture "oh this part here is pretty" or "this section caught my eye." He may open up if he's not expected to explain what he drew, but respond open ended to your observations without feeling pressure (not to say you're pressuring him, only that he might feel pressured).
My son has language impairments- he cannot explain the plot of a book in detail when asked directly. His standardized testing puts his reading comprehension at the 12th grade level (he's ten)- it's not understanding- it's verbal synthesis (breaking down the entire content of a book or drawing and explaining it).

OP here, just to clarify: some of his drawings and writings are incoherent, baffling and totally unclear to his parents. I'm never critical of him about that and I don't expect the Sistine chapel. Just concerned that he is doing things that evade his understanding as well as ours.

I'll say up front that my kids don't have autism, so I am not the most educated person on this forum. But don't autistic kids sometimes mimic lines from movies, TV shows, etc? Maybe this is similar but with imagery? So maybe he sees foreign language characters or other images in a book or graphic novel and can memorize them easily, and it comes out in his art work even if he doesn't fully understand it?

I would look for blogs of adults with autism! Creating detailed fantasty/alternative worlds is not uncommon. Finding a way to engage with your child about his world might be helpful.

https://thesilentwaveblog.wordpress.com/2017/01/17/creating-an-alternative-world/

He might have difficulty accessing the language necessary to explain things. Difficulty using language to communicate effectively is one of the hallmarks of autism.

My guess is that he really just doesn't want to explain them to you, or they're not representational. Sounds like he's got a good imagination.

My DS is 6 with an ASD level 1 DX. He loves, loves, loves drawing, writing, "making books", and reading. I count my blessings there but the problem is he is often guarded and evasive when I make the most casual of queries like, Hey what is that you drew, can you tell me about your story, etc. I have other older kids and know that 6 year olds don't often provide incredible narrative explanations but I'm concerned by his answers. He often hides his work, says it's "too hard to explain," or more often appears genuinely confused by what he drew or wrote.

His handwriting is legible but a lot of his drawings resemble symbols or foreign characters. He has no foreign language exposure.

I want to give you more reason for ridicule so I'll just admit I'm afraid he has some additional condition, in addition to ASD, that doesn't leave him in full control of his mind all the time. What would cause him to draw and write dozens of drawings and ages that don't make sense to him?