It could be a habit, or maybe the horse thinks it has time so it'll get back to it later in time. Make sure they're up to date on teeth and make sure the horse is getting a reasonable diet sometimes the horse could be getting to much or just something they don't like to eat.
Is it a actually a "habit"? Have you asked the previous owners? Maybe they had him in different food if he is a picky eater? That's the first place to start. Then check teeth. How much food is he getting? What season are you in because sometimes if the grazing they have access to is far more desirable then they are too satiated. Perhaps he is pining for his previous yard? Maybe he needs a companion? Have you checked for low grade fever? There are many reasons why a horse doesn't eat his food. My first reaction will be to check temp, heartrate and RRR-I've had the weirdest reasons for lack of appetite.