Could it be ringworm?  My horses had this in the past and it started with one patch and then went on to several in a very short space of time.  The vet can give you a solution called mycophyt which you dilute in water and wash the patches with, you've also got to treat their equipment such as brushes and bridles etc so that you are getting rid of the infection.  It's not and actual worm, more a fungal infection usually caught from cattle.  If there are cows nearby e.g. in the next field and he's had a sniff at them over the fence that might be your answer.