What an awful thing t find hun. You are upto date with tetanus etc arent you? If not, get them done.
Now...it may not be as bad as it looks. I have treated worse, (Im a trainee vet, and qualified equine vet tech). It looks sooo bad at the moment because its fresh, swollen, dry blood, etc. You need to ensure you get an antibiotic shot, and get some anti inflams from your vet, bute is good for rapidly reducing swelling, its is also a painkiller. Although you will probably get some scarring, once the swelling goes down it may not be as bad as it first appears, swelling makes thing look much worse.
Keep it clean as possible, trckle cold water over it to aid in reducing the swelling too...i know its easier said than done, but this will also improve blood flow so ensuring the white blood cells attack any incoming infections.
It will quickly improve, don't be too worried, but that's easer said than done too.
Hope he gets better soon.
A horse belongs in a stable not on a table!