Oh...that's sounds like a hard deal...poor guy.
There seem to be a variety of feed-through fly controls. One has the apple cider vinegar in it along with other ingredients. I've also heard of ones with garlic powder in them. Some brands include Inside-Out by Cheval Int. , Garlic by Equine Science, and Get Away by Jeffers.
I don't know how well any of them work but I am thinking about trying one myself.
I hope things get better for Toby!! Good luck!!