Maybe. But how long can the person who knows the private key, or knows how to get the private key, or knows how to derive the private key, or knows who the other signatories of the multi-sig keys, how long can he hold out?

What will it take to break him?

And if there is no practical hope to get the bitcoins, how long before his captors realize that? (They may even refuse to believe you. So you're screwed if you actually can't get the bitcoins.)