You see this most of the time with foals. They do it b/c they need the benifical bacteria. Go and get a couple big tubs of yogurt and mix it in with his feed. That helps most of the time if that is the reason. If it is one of the others listed then you will need to try some other things.
Also just b/c there is a salt block does not mean they are getting the minerals they need. Solid blocks are hard for the horse to digest and to get enough. I use loose minerals myself along with blocks. That way I know they are getting what they need.
my horse, and one other horse along with him get 6-8 flakes spread around plus 1 and 1 half quarts of grain twice a day! he is the one who usually steals the 3 year olds food. he isn't my horse so i cant give him anything and my friend just gets mad when i say she should do something about it!
Could be to replace beneficial bacteria or to get some nutrient missing in the diet. Any signs of ulcers? Any course of antibiotics done lately? I would be feeding trace minerals as suggested. Know that horses are grazing animals. Unlike us, they are producing stomach acid 24/7. If there is no hay going through and the horse is waiting for his next meal, then the stomach is eating itself while he waits. Slow feeders are also a good regulator and fight boredom. Salt and mineral blocks don't cut it and neither yogurt. A proper probiotic is a good addition to any diet.