You have to look if he has an appetite, if he is concentrated and applied in his work, if when you ask him to do an exercise he is voluntary. Does he develop tics or else he loses his appetite. Is he happy when he sees other horses. It should not be left alone. In summary all the signs that seem abnormal in your behavior are to check :)
I hope this helps
Keeping his head low, looking down, staring at the wall, not responding when called... If he's ever been happy, you can easily note he's depressed!
About "cure", let the horse run free in a field, opt for making him spend most time possible outside, interact with other horses and animals, give him more attention and affection.
Hope it helped!!