One of my coworkers was blind. With an screen reader, he used Excel, LaTeX and other computer stuff, but IIRC he didn't program.

I'd recommend to skip Scratch and clones. Jump directly to Python. The indentation may be a problem, so you must find out how to configure the screen reader to say this invisible spaces of the indentation. Perhaps it's better to use tabs for the children, because they are easier to count. (Some editors show a small arrow instead of a tab, so I'm optimistic this is fixable.)

I recommend t create a small library to play sounds for the kid, so he can use it directly without caring about the nasty details. Something like

  sound("thunder.wav", left = 80, right = 20)

My idea is that he can make a small game where you walk and hear the sound in each room of a (haunted?) house. The volume and direction can gives cues. (The code to handle arrows may be tricky too, perhaps encapsulate it in another library.)

(When my daughter was young, I made a 3D library for red-blue glasses. He used it do draw simple objects and buildings (something like the front of the Parthenon with square columns).)

If he is old enough and likes math, perhaps he can learn to nest a few `for` and `if` and bruteforce some elementary math problems. I'm not sure if the screen reader is good enough for this. Perhaps a text to downloaded speech library may be useful, so he can use

  say("The result is", r)

instead of

  print("The result is", r)