Scratches in the winter always means more work
If you have a place to get him inside, I would:
1) Take my least favorite blow dryer to the barn.
2) Shave most of the hair off his legs, including fetlock hair, coronary band, and the "seam" down the back of the legs starting from the hock down.
3. Wash his legs in the medicated shampoo.
3.1 Towel dry.
3.2 Finish drying with the blow dryer on a low setting.
If the scabs want to come off, take them off. If they are difficult to remove, leave them on because things are not healed under the scab yet and the sore will bleed when the scab is picked. The horse won't bleed to death, it just makes a bigger mess to try and put medicine on.
Creams and ointments are great but, depending just how cold your area is, they might make a big mess that will clump up and be tough to clean the next go-round<---- which should be once daily but twice would be better if the scratches are really bad.
I have a mix of tetracycline, DMSO and ? the vet gave me when two of my horses tangled either with an ant hill or a condominium of chiggers.
I use that if it's too cold. Point-being, maybe the vet could mix something for you?
If DMSO is involved, the legs do have to be thoroughly washed (and you need to wear surgical gloves) because DMSO is a carrier and carries anything into the skin with it.
Hope this helps