Welp, tried for 2 days any kind of break/stun abilities, I still can't break/interrupt any of the Old King's casts... I guess I will forget about it for the moment.
I had the same issue for a while. Going from my experience...
You can only break Old King if he's charging Zexcalibur - the time where he has an icon over his health bar. I don't know the exact reason it didn't work before, but it might be an agility thing. That's the best guess I have.
Assuming there isn't a stat requirement (and I don't think there actually is) or you meet them, it's a two step process: dodge N-Field Gravity - you'll still get hit, but you won't get the debuff that reduces your speed to 1% - and then break him on the follow up while he has the book icon.
I know it seems obvious, but, again, I struggled with him for several hours.