good plan, let us know if it alters.

sounds like it's rain-scald, wash him frequently with hibi-scrub and warm water (leave the soap on for 5 minutes to take effect). This is easlier said than done - my mare has rainscald but because of the awful cold and windy weather i haven't been able to wash her for a couple of weeks. Luckly the first couple of times i shampooed her it worked well and its clearing up well.

Ensure that the rug is cleaned frequently and re-waterproofed to ensure its up to standard.

The only other thing it could be is ringworm - shampooing with hibi-scrub would be good for this too. Check the other horses in the field - it'll be obvious if they are the same. Otherwise treat it as rainscald.