From your description my first instinct would have been Autism. What factors ruled it out?

Does your son understand what you say, even if he can't form words you can understand? What about sign language? Can he master Yes and No type concepts in sign?

Have you had his hearing checked?

Can he hear simple words like cat, dog, fence, and milk, and then point to pictures of the corresponding item or creature?

What about computer interface usage? Can he play simple children's game on an electronic device on his own or with an explaination from you?

In my personal case, my challenge with reading is my light sensitivity. Without my Irlen glasses, I have problems with reading a block of text or small print. The Irlen lenses block out the wavelengths of light that irritate my brain so much that words become incomprehensible. Earlier today I was attempting to read a professional paper written by a friend, but she and another friend were so noisy I couldn't concentrate. I read the same word over 30 times. Once I asked them to stop talking for a little bit, I was able to get into the paper and when they started talking again it became more difficult.