I had a dog once in a sort of similar situation. She had cancer, and it meant she couldn't eat. She was old (13), and there was the possibility it might have spread. I vividly remember sitting in my parents' living room and 'discussing options'. My mum wanted to put her down, I didn't. I said, go for the operation, if it doesn't go ok, we gave her a chance and she'll never wake up. If it does, she might have a few years yet. It was a lot of money, but we could afford it. The discussion went on for some time, and I have no idea how, but somehow she understood something of what we were talking about. She'd always been my mum's dog, always, yet she came over to me and didn't want to be near her. Wouldn't look at her, anything. We had the op in the end, and afterwards she didn't want to be near my mum, didn't want her with her, just turned away, so I stayed with her that first night. The operation gave her two weeks before we found the cancer had spread to her spine. She died very quickly, but she did have two lovely pain-free weeks to enjoy. To this day I firmly believe we did the right thing. She was not ready to just give up and go peacefully. In contrast, a friend's horse had cushings. Over one long winter, she quite blatantly held on for her owner, who she'd been with since she was a foal. She broke a hip, and recovered, then went down again one day and really struggled to stand. She looked so tired, so ready to just give up and have some peace. She was put down and possibly would have been ready for it sooner, but she held on for the girl who had loved her all these years. When they are ready to go, the fight goes out of them. They look tired, and fed up with life. For your horse, if getting up becomes a real problem, you're left with trying that operation if it might work, or with PTS. What determines that, is finances obviously, viability and his quality of life.