Last time he had the hair loss, did you get a vet out?
Does he have any known allergies? To food or any plants? Is he out with any other horses that have hair loss (either before or after he got his?)
Has anything changed in his routine (new pasture, new pasturemates, new food, new barn?)
Was any of this different from last time he lost his hair?
If you didn't have a vet out, I would suggest to call a vet out so he/she can do some skin scapings or something and figure out what is causing the hair loss.