I've attended. It was interesting but probably never again due solely to potentially dangerously wedged in crowds. At one point some young girls panicked and had to be pulled the side of a house into a window. It really is that tight. Of course you could try and stay out of the actual "centre", but in that centre it truly is as tight as the tightest tube carriage you've ever stood in, and moving at a shuffling pace trying to stay calm.
I am no expert on autism, but unless the condition makes the chap calmer, non-claustrophobic and noise tolerant (and maybe it does, I really don't know), I would give it a wide berth.
To be honest you, or him, may not have experienced anything like it before, and I don't recommend it as a place to experiment, since there's basically no way out.