If he provides the two-key option, then he is definitely trustable...

I'm probably saying something that everyone here already knows, but:
The two-key coin requires both the private key on the hologram and your secret key to redeem it.  This means that
1. Since the address of the coin is easily gotten from your two public keys, you can verify casascius sent coins there.
2. He has no incentive at all to put an incorrect private key onto the coin.
3. Thus, you and only you can redeem the coin.