Unless there's some language in the contract that you can use, that makes his arrest a breach of the contract on his part, you still have to pay the rent for the 6 month lease.  Or, if there's something he's obligated to do on his side, during the six months, that he doesn't get done, again you can walk away if he's in breach of contract.

Even if he were convicted, that probably wouldn't be grounds for breaking the contract.  And right now, until proven guilty, he's still innocent.