I believe you said you already have photo documentation, correct? Great! Move your horse asap. Do not forewarn the BO, just get a trailer out there, a few friends to help load your stuff, and leave.
The BO is not fulfilling his end of the contract. The chances of him going to court and winning an order forcing you to pay are slim, especially with a "vague" contract, as you called it. You can pay him for one more month, that will cover a 30 day notice, which is far more standard in a boarding contract.