Oh what terrible news!!!

My friend has an ACD, an he is tireless--he runs and runs and runs and runs--I'm sure just like your Casey!!

I know you have talked to the refuge farm and the vet, but maybe speak with a trainer. Has Casey had basic obedience?? Maybe he just needs a tune up on his manners, and you could look into doing agility or something with him??
Is there any way he could go to a farm every day or so to get his "work" done, and still come home with you??

This is heartbreaking news, I can only imagine how you feel. I do understand that you are trying to do what is best for him though, good luck.