Things you can try :
- worm him, might also need to give him a flukicide
- stop the feed, wethers won't need feed as well as grass or hay especially at this time of year and especially primitives
- have himalyan rock salt always available, and give them a drench of a good chelated mineral drench once or twice a year
- reduce the amount and/or richness of the grass. Soay are more used to "severely disadvantaged" type ground, rushes and heather with poor moor and marsh grasses.
- put them on bare ground and feed very poor hay, and not much of it