I've seen wounds that bad & worse heal nicely with dedication from the owners to keep the wound treated & clean. I would imagine the reason he isn't putting weight on that leg is because it stretches the wound and causes pain. That is a huge area of damaged skin. I'm no vet so this is only from my experiences with my horses & others I pet sit for. The owners often call me in to care for their horses while they are at work & I get a lot of information directly from the vets.
If in doubt, call your vet.
Keep the infection away with the antibiotics; The hydro (hosing with cool water) is also a very good idea. Let the area dry before medicating & wrapping if you choose to keep it wrapped.
I like to use Shreiner's Herbal Solution on all wounds, including ones as bad as your horse's. It helps keep the flies away but the herbs work wonderfully on wounds.
http://www.schreiners.com/
If the wound starts to dry out too much and crack - by leaving it unwrapped, you may want to use some Fura-Zone.
Ideally, I'd hydro, let dry, cover in Fura-Zone, and wrap using a non-stick pad over the oozy part of the wound, followed by soft clean cotton over the entire area, and held in place by vet-wrap. I'd do this for at least 3-4 days. You will see improvement and will know when you can stop wrapping. Then I'd switch to the Schreiners Solution: Hydro, let dry, apply Schreiner's, & leave the wound open.
The trick to wrapping, besides wrapping in a manner that makes the bandage stay in place, is making sure you keep enough soft cotton over the wound so the vet-wrap doesn't irritate the wound & area where there is no hair. Most people don't believe it, but vets assure me that it is possible to get vet wrap too tight and restrict adequate blood flow to the area being wrapped. You need good circulation to that area so the wound can heal.
Sure hope your horse gets better!
PS: Vets have also told me not to use Betadine or Iodine after the first 24 to 48 hours of the occurrence of a wound. It kills the new skin cells forming to heal the wound.