Squishypuff wrote:
I started the apple cider vinegar in his diet...

That shouldn't make his droppings loose...does he have lots more green grass to eat now that you've built your new fence? Lush green grass & eating more than he is used to will cause colic & loose droppings & can cause founder. This is the direction I'm kinda seeing from your post on finishing your new fence & the symptoms of your horse's problems. Also if he ate on a fruit tree or plant that didn't agree with him that may be planted in your yard will cause the same reactions as from your pics it looks like he has free access to your yard & many ornamental plants are poisonous to horses.