Stuck in dangerous boarding facility!

If everything is as bad as you say, take a ton of pics before your meeting with him and then show him that you have the proof of the declining conditions. If he refuses to do anything, I'd quietly find another barn, move my horse and then tell him to take me to court for the remainder of the contract. I doubt he'll follow through, but if he does, you have your photos.

Also, if you do end up moving, take photos of the new place showing how superior it is in every way to the current one, so that if you have to you can show that in court as well.

The BO drives the tractor into the pasture and the son and son's girlfriend are to keep the horses away from the tractor while they bring in the hay. Usually they just shoo them but at one point he decided it would be easiest to throw things at them instead... brilliant.

The other boarder and I are going to secure new facilities for our horses and then get them out asap. We do have the photo documentation to show negligence by the BO that creates an unsafe environment. We will offer to pay the next month's board to cover not allowing 30 days for notice and will have all of the documentation in order in case he chooses to pursue filing against us

Thank you everyone for the advice!

Get your horse out now, then go to the BO and show the photos and tell him why you have left. If you go to the BO first, and tell him how upset and that you have photo, then you could end up with more issues if he thinks he is losing a boarder.
I would not keep a horse at a place that was not safe and as soon as I saw the BO's kid throwing rocks at the horses, I would have packed up that day. Next time a rock could hit your horses eye and put it out or some other issue.
Move the horse, don't pay another dime and let him take you to court.

Take pictures, send a certified letter ( keep copy and reciept) and copies of pictures, say the safety issues need to be fixed immediately, and other issues need to be fixed by "X" day. Or you consider him in breach of contract and for the safety and well being of your animals I will be leaving on such and such a day.
I am going to guess it would work like a house rental. You have to notify the landlord of the issue, and then give a reasonable amount of time to fix it. At that point you can pretty much just leave if the problems arnt fixed.
Knowing me I wooulda said along time ago hay you need to fix this stuff. I wouldnty have let much time pass before I moved out. Want me to pay till May ? Take me to court, I have photos of the unsafe conditions.

What does your contract say about giving notice to leave? Can you type it out here?

The second that my horse was fed moldy hay and rocks or other things throw at them and any other lack of care I would have been gone without warning that very day.

Load horse in trailer. Load your horse belongings in trailer. Drive away. Take this posting to court 45 days later if the owner has the nerve to try and enforce the contract.

He voided the contract by being negligent. You do not have to stay. You just have to be willing to give your side in court when he comes after you. If the other boarder goes with you, there is more power on your end.

Document everything. Photograph all evidence of negligence. Then leave.

Personally, if I had any knowledge that a rock was thrown near my horse to move it, I would be in jail for the butt-wooping I gave the rock thrower.

I once ignored a nagging feeling I had that something was wrong. It cost me my dog's health. Trust me. If you feel you have to post it here - it's too late. You are failing to listen to your nagging feeling.

Load horse. Leave. He can't stop you from taking the horse off the property. He can only force you to pay.

Through the courts.

The courts will not side with his negligence if you can show any proof, such as photos. They may make him bring the place up and you return, but you can also argue the safety of the animal through his anger.

The contract doesnt specify anything about wanting to leave. It just says that if you choose to leave before the contract is up you will pay the remaining board up until when the contract would have expired, which in this case is the end of May.

Why in the world did you sign a contract that goes for at leat 5 months?? I would still leave immediately and then worry if he takes you to court, which I doubt he will since you will also contact him afterwards and tell him you have photos and proof of care that is detrimental to your horse. If you stay, you are risking your horse's health.