My horse got a bad leg wound last year. Not even in the ballpark as bad as yours, but the vet couldn't stitch him either.
However, there was a LOT the vet could do for him.... antibiotics, both fast acting injection and oral for 10 days. Injection for pain, as well as a weeks supply oral. Tetanus shot, even though he was up to date on his annual.
If your vet did not advise any of that treatment, maybe you should take to heart what he said.. There is nothing he can do. I love my horse beyond words, and I can't imagine the pain of putting him down, but I would rather that than watching him die of major infection. If your horse hasn't had any antibodies yet, don't even know if a massive dose would even help at this point.
I am so very, very sorry for your poor horse. What a horrific thing to have happened.