Wow that looks awful! Any idea how he did it?
How long after it happening did the vet see it? I'm kind of surprised your vet didn't prescribe any bute or antibiotics. Or tidy it up. Its a huge wound and very open to dirt and flies which are highly likely to introduce infection to it. My horse just had a 3" cut and has had bute and 2 different antibiotics just for that! Keep it as clean as possible- try cold hosing it every day to wash dirt out and maybe bring some swelling down.
Wouldn't like to guess on a healing time, especially seen as it's likely to go septic atm :/
We lose ourselves in the things we love, we find ourselves there too ~Kristen Martz