First of all you need to look at his bedding, find a softer place for him to lie down (when he's in contact with the ground in stall or field, this is what usually causes hock sores).
Then, there is nothing much you can do about it, it is mostly prevention like i said. Once it's there, you can clean it with iodine, put an antibiotic cream (ask your vet for a special brand that will be really in harmony with your horse's problem), you can also do hydrotherapy 3 times a day during 10minutes.
Older tip: use honey, this is good for healing and has an antiseptic power.
You can cover the sore only if it's open with hock boots overnight.
I hope I could help you, don't hesitate to contact your vet for more specific advices :)