I can't count the number of times I have dealt with scratches on clients horses.
I have seen it get bad enough to cause the horse to be lame. One case the even worse than that.

This is how we treated the horses I dealt with.

Clip the hair as short as possible and clean the area with a betadine scrub. Dry completely. Just make sure to not nick or give the horse clipper burn when clipping the leg/pastern area as you don't want to cause a secondary infection.

Then we would have them apply a ointment that our clinic made (consisting of Desitin, furazon, deximethasone). We would have them apply once daily until healed and DO NOT WRAP LEGS. It is important that you leave the legs uncovered. Just wipe them off and apply new each day.

Some horse do get bad enough that they will need to either be put on an oral and/or topical antibiotic for 7-10 days.

The horse needs to be in a dry pen during treatment.

If it reoccurs you may need to check his enviroment, tall grass/weeds in the pasture, mud holes, dirty stalls can all be contributors to the horse getting scratches again. If it continues to occur after the above has been done talk to your vet about photosensitization.

Talk with your vet

Hope this helps