Hi S.. My daughter also has mild autism. She struggles with comprehension and social skills.
I taught her to read by repetition. She may not fully understand what she reads but by repetition she learned her letters and sounds.
If your son can learn numbers, he can learn letters. But, it will take a whole lot of repetition because the comprehension may not be there.
I began teaching my daughter her letters and sounds through a music video from leapfrog. I believe it was called The Letter Factory. It is so good that while I was teaching her my 2 year old learned all his letters and sounds within a few months. He was learning to read right after.
Find out what triggers your son's learning buttons. Is it music, video games, or just plain repeating yourself?
Throughout the day find a letter and point it out and say it's sound. Start with A and after he has it memorized then on to B. Don't rush him.
My daughter learned her numbers quick so math came easier for her. She memorized them so much that if a person gave her their phone number she would memorize it. Unfortunately time was never an issue for her so these people got phone calls in the middle of the night. Hahaha.
Take care and enjoy being creative with teaching him. God Bless~