Help with horse and loose stool?

I'm going to guess by your info that he's pretty old. The reality is, is that old TB's are hard to keep and just slowly start sliding down hill. They loose the ability to digest food well and this leads to weightloss and a whole other list of problems. If his stools stay loose or get looser I'd call the vet. Did he have a fecal egg count done? Worms and other parrasites are possible and would explain his weight loss. When's the last time he was floated? Bad teeth can lead to weight loss and bad stools. I'm hoping the vet is able to figure out something soon for you. :D