I’d ask him what I’d ask anyone.

What was the biggest mistake you made that you found most difficult to face up to and admit to yourself?

In this sense it would be in terms of ‘philosophical’ ideas - but I’d be interested to hear his general ‘human life’ answer too.

On a personal note, my own ‘mistakes’ would include dismissing areas of knowledge I found to be ridiculous/nonsense on the surface - especially psychology and more recently (just five years ago) economics. Generally the things in life most ‘repulsive’ turn out to hold a whole load of juices morsels that help seemingly unrelated ideas/thoughts/questions flourish more readily.

I'd really like to ask him why he rejected Godel's Incompleteness theorems as, what he called, "logical tricks".

What I would have liked to ask him is whether he saw the limits of speech in some parallel to Taoism where what cannot be expressed is the beginning of a journey and not a result after many attempts to the contrary.

I am not sure if my comments are welcome.
If not, I will stop.