I Have A Pony With Chronic Diareah, Blood Work Is

I have a pony with chronic diareah, blood work is normal, ultrasound of all organs is normal, we have been dealing with it since March. He is currently at a homeopath's as the vets have given up. It appears he has been poisoned..... Any suggestions? Prednisone helps but is not a long term solution, could be arsenic poisoning from chewing pressure treat wood... Or a reaction to a dewormer. Not sure.