I don't think it is sweet itch. Cause he isnt rubbing him self raw or anything(doesnt luk like it from pics).
My horse had something similar twice. Once I used savalon. And the other time first I got a soft toothbrush and hibiscrub waterd down and got all the dead skin off and then I put athletes foot cream on it.
Both of what I did worked but the savalon didnt work as fast as the other stuff.