I just had my vet out after dealing with our pony's below knee laceration once I saw it wasn't healing.
He said use Saline - you can use the children's saline solution for nasal passages, but make sure it is just the saline, not the medicated kind, instead of water and then follow with betadine to clean it.
I asked if I should have had him out sooner (it was about a week later when he saw it), and he said there was nothing he would have done at that time except recommend saline verses water and wait to see if it looked infected to call for anti-biotics. . .
As it was, it did get infected, and she is on an oral antibiotics.
He said to keep it wrapped and apply a topical antibiotic cream and to only change it every 3 days or so.
Also, is your horse UTD on tetnus? If not you need a quick acting tetnus shot given.