Vielleicht:
to break through sb.'s reserve

He wouldn't be drawn
I tried to draw him

@Jack
I like your verb "draw". That's the word I was trying to recall when I made my first suggestion: to draw s.o. out of his shell. This is an idiomatic expression that is still quite common.

You can also use the verb "draw" without "of his shell", but you must keep the particle "out": to draw him out. Otherwise, it means "zeichnen".

I could tell that Mike had something on his mind. I put him as much at ease as I could and drew him out by asking non threatening questions.

To "draw someone out" can be in any one of a number of situations, formal or informal. I believe its origin may be "to draw someone out of his shell" i.e. of self-protection and the image likely comes from a tortoise.