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.
A horse at my barn sometimes does the same thing. Normally it's because it's very hot or cold, or because of stress and anxiety.
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.
Mix some molasses in with the grain
Thanks for all the good suggestions! Just to add some more info: he has two buckets of water relatively full 24/7. He gets beet pulp and Q12%
Could be to much feed at a time. You could try adding a little molasses or soak his feed to make it softer and easier to eat. Another thing is always provide water while they are eating grain.
Consult with your vet. Sometimes, teeth, type of feed or quantity of feed plays a factor. However, some horses will stop eating if there is a physical problem in which pain is caused. Hope that helps!
Maybe you're giving him too much food? Or maybe try putting in a little Stock and Stable Sweet Feed at the bottom of his grain to get him to finish?