From what I have read, the best thing to do with sarcoids is leave them alone, and if that is what your vet recommends you should listen to him/her. As for the coat, rather than buying supplements perhaps you should review his nutritional needs and perhaps change his feed. At this time of year, hay quality can be iffy, so maybe adding some hay cubes (I use timothy rather than alfalfa) would help. You don’t say how old he is, but needs change with age as well as with conditions. Also, thorough daily grooming is a must for a healthy coat.
It is never the horse's fault